added data_type_traits for enums

# Conflicts:
#	demo/main.cpp

.gitignore

@@ -1,3 +1,4 @@
 .idea
 cmake-build-debug
 Testing
+debug

CMakeLists.txt

@@ -22,6 +22,10 @@ find_package(SQLite3 REQUIRED)
 find_package(PostgreSQL REQUIRED)
 find_package(MySQL REQUIRED)
 
+message(STATUS "Found ODBC config ${ODBC_CONFIG}")
+message(STATUS "Adding ODBC include directory: ${ODBC_INCLUDE_DIRS}")
+message(STATUS "Adding ODBC libs: ${ODBC_LIBRARIES}")
+
 message(STATUS "Found SQLite3 ${SQLite3_VERSION}")
 message(STATUS "Adding SQLite3 include directory: ${SQLite3_INCLUDE_DIRS}")
 message(STATUS "Adding SQLite3 libs: ${SQLite3_LIBRARIES}")

Todo.md

@@ -4,3 +4,44 @@
 - Read in entity fields
 - Add special handling for update in backends
 - Add ODBC/SQL Server backend
+
+
+Fetch eager strategies
+======================
+
+ONE TO ONE/MANY
+*person*             *address*
+- has one address    - belongs to person
+
+=> join "address" on "person.id" == "address.person_id"
+
+*address*             *person*
+- belongs to person   - has one address
+
+=> join "person" on "address.person_id" == "person.id"
+
+*book*               *author*
+- belongs to author  - has many books
+
+- => join "author" on "book.author_id" == "author.id"
+
+HAS MANY TO ONE (WITHOUT RELATION TABLE)
+
+*author*             *book*
+- has many books     - belongs to author
+
+- => join "book" on "author.id" == "book.author_id"
+
+if "has many" type has primary key & field "author_id"
+if table name belongs to entity template type?
+
+HAS MANY TO MANY (WITHOUT RELATION TABLE)
+
+*student*            *student_course*           *course*
+- has many courses   - belongs to student
+                     - belongs to course        - has many students
+
+=> join "student_course" on "student.id" == "student_course.student_id"
+   join "student_course" on "course.id" == "student_course.course_id"
+
+if has many type hasn't primary key (is relation table)

backends/mysql/test/CMakeLists.txt

@@ -3,7 +3,7 @@ Include(FetchContent)
 FetchContent_Declare(
   Catch2
   GIT_REPOSITORY https://github.com/catchorg/Catch2.git
-  GIT_TAG        v3.4.0 # or a later release
+  GIT_TAG        v3.5.4 # or a later release
 )
 
 FetchContent_MakeAvailable(Catch2)

backends/postgres/test/CMakeLists.txt

@@ -3,7 +3,7 @@ Include(FetchContent)
 FetchContent_Declare(
   Catch2
   GIT_REPOSITORY https://github.com/catchorg/Catch2.git
-  GIT_TAG        v3.4.0 # or a later release
+  GIT_TAG        v3.5.4 # or a later release
 )
 
 FetchContent_MakeAvailable(Catch2)

backends/sqlite/include/sqlite_prepared_result_reader.hpp

@@ -32,7 +32,7 @@ public:
   void read_value(const char *id, size_t index, char *value, size_t s) override;
   void read_value(const char *id, size_t index, std::string &value) override;
   void read_value(const char *id, size_t index, std::string &value, size_t s) override;
-  void read_value(const char *id, size_t index, sql::any_type &value, sql::data_type_t type, size_t size) override;
+  void read_value(const char *id, size_t index, sql::value &val, size_t size) override;
 
 private:
   sqlite3 *db_{nullptr};

backends/sqlite/src/sqlite_prepared_result_reader.cpp

@@ -107,8 +107,8 @@ void sqlite_prepared_result_reader::read_value(const char *id, size_t index, std
   query_result_reader::read_value(id, index, value, s);
 }
 
-void sqlite_prepared_result_reader::read_value(const char *id, size_t index, sql::any_type &value, sql::data_type_t type, size_t size)
+void sqlite_prepared_result_reader::read_value(const char *id, size_t index, sql::value &val, size_t size)
 {
-  query_result_reader::read_value(id, index, value, type, size);
+  query_result_reader::read_value(id, index, val, size);
 }
 }

backends/sqlite/test/CMakeLists.txt

@@ -3,7 +3,7 @@ Include(FetchContent)
 FetchContent_Declare(
   Catch2
   GIT_REPOSITORY https://github.com/catchorg/Catch2.git
-  GIT_TAG        v3.4.0 # or a later release
+  GIT_TAG        v3.5.4 # or a later release
 )
 
 FetchContent_MakeAvailable(Catch2)

backends/tests/QueryTest.cpp

@@ -370,7 +370,7 @@ TEST_CASE_METHOD(QueryFixture, "Select statement with foreign key and for single
 
   auto expected_flight = flights[0];
 
-  REQUIRE(result.has_value());
+  REQUIRE(result);
   REQUIRE(result->id == expected_flight->id);
   REQUIRE(result->pilot_name == expected_flight->pilot_name);
   REQUIRE(result->airplane.get());
@@ -456,43 +456,77 @@ TEST_CASE_METHOD(QueryFixture, "Select statement with many to many relationship"
   }
 
   auto result = db
-  .query(schema)
-  .select({"r.id", "r.name", "ri.recipe_id"})
-  .from({"recipes", "r"})
-  .join_left({"recipe_ingredients", "ri"})
-  .on("r.id"_col == "ri.recipe_id"_col)
-  .fetch_all();
+    .query(schema)
+    .select({"r.id", "r.name", "ri.ingredient_id"})
+    .from({"recipes", "r"})
+    .join_left({"recipe_ingredients", "ri"})
+    .on("r.id"_col == "ri.recipe_id"_col)
+    .fetch_all();
 
+  std::vector<std::tuple<unsigned long, std::string, unsigned long>> expected_result_one_join {
+    {7, "Apple Crumble", 1},
+    {7, "Apple Crumble", 4},
+    {7, "Apple Crumble", 5},
+    {8, "Beans Chili", 6},
+    {8, "Beans Chili", 7},
+    {9, "Fruit Salad", 1},
+    {9, "Fruit Salad", 2},
+    {9, "Fruit Salad", 3}
+  };
+  size_t index{0};
   for (const auto &r: result) {
     REQUIRE(r.size() == 3);
-    std::cout << "record r.id " << r.at(0).as<unsigned long>().value() << " r.name " << r.at(1).as<std::string>().value() << " ri.id " << r.at(2).as<unsigned long>().value() << "\n";
+    REQUIRE(r.at(0).as<unsigned long>().value() == std::get<0>(expected_result_one_join[index]));
+    REQUIRE(r.at(1).as<std::string>().value() == std::get<1>(expected_result_one_join[index]));
+    REQUIRE(r.at(2).as<unsigned long>().value() == std::get<2>(expected_result_one_join[index]));
+    ++index;
   }
 
   result = db
   .query(schema)
-  .select({"r.id", "r.name", "ri.recipe_id", "i.name"})
+  .select({"r.id", "r.name", "ri.ingredient_id", "i.name"})
   .from({"recipes", "r"})
   .join_left({"recipe_ingredients", "ri"}).on("r.id"_col == "ri.recipe_id"_col)
   .join_left({"ingredients", "i"}).on("ri.ingredient_id"_col == "i.id"_col)
   .fetch_all();
 
+  std::vector<std::tuple<unsigned long, std::string, unsigned long, std::string>> expected_result_two_joins {
+    {7, "Apple Crumble", 1, "Apple"},
+    {7, "Apple Crumble", 4, "Sugar"},
+    {7, "Apple Crumble", 5, "Flour"},
+    {8, "Beans Chili", 6, "Butter"},
+    {8, "Beans Chili", 7, "Beans"},
+    {9, "Fruit Salad", 1, "Apple"},
+    {9, "Fruit Salad", 2, "Strawberry"},
+    {9, "Fruit Salad", 3, "Pineapple"}
+  };
+  index = 0;
   for (const auto &r: result) {
     REQUIRE(r.size() == 4);
-    std::cout << "record r.id " << r.at(0).as<unsigned long>().value() << " r.name " << r.at(1).as<std::string>().value() << " ri.id " << r.at(2).as<unsigned long>().value() << " i.name " << r.at(3).as<std::string>().value() << "\n";
+    REQUIRE(r.at(0).as<unsigned long>().value() == std::get<0>(expected_result_two_joins[index]));
+    REQUIRE(r.at(1).as<std::string>().value() == std::get<1>(expected_result_two_joins[index]));
+    REQUIRE(r.at(2).as<unsigned long>().value() == std::get<2>(expected_result_two_joins[index]));
+    REQUIRE(r.at(3).as<std::string>().value() == std::get<3>(expected_result_two_joins[index]));
+    ++index;
   }
 
   result = db
   .query(schema)
-  .select({"r.id", "r.name", "ri.recipe_id", "i.name"})
+  .select({"r.id", "r.name", "ri.ingredient_id", "i.name"})
   .from({"recipes", "r"})
   .join_left({"recipe_ingredients", "ri"}).on("r.id"_col == "ri.recipe_id"_col)
   .join_left({"ingredients", "i"}).on("ri.ingredient_id"_col == "i.id"_col)
   .where("r.id"_col == 8)
   .fetch_all();
 
+  index = 3;
   for (const auto &r: result) {
     REQUIRE(r.size() == 4);
-    std::cout << "record r.id " << r.at(0).as<unsigned long>().value() << " r.name " << r.at(1).as<std::string>().value() << " ri.id " << r.at(2).as<unsigned long>().value() << " i.name " << r.at(3).as<std::string>().value() << "\n";
+    REQUIRE(r.at(0).as<unsigned long>().value() == std::get<0>(expected_result_two_joins[index]));
+    REQUIRE(r.at(1).as<std::string>().value() == std::get<1>(expected_result_two_joins[index]));
+    REQUIRE(r.at(2).as<unsigned long>().value() == std::get<2>(expected_result_two_joins[index]));
+    REQUIRE(r.at(3).as<std::string>().value() == std::get<3>(expected_result_two_joins[index]));
+    ++index;
   }
 
   db.query(schema).drop().table("recipe_ingredients").execute();

backends/tests/SessionTest.cpp

@@ -5,6 +5,7 @@
 #include "matador/sql/session.hpp"
 
 #include "models/airplane.hpp"
+#include "models/flight.hpp"
 
 class SessionFixture
 {
@@ -16,9 +17,8 @@ public:
 
   ~SessionFixture()
   {
-    drop_table_if_exists("flight");
-    drop_table_if_exists("airplane");
-    drop_table_if_exists("person");
+    drop_table_if_exists("flights");
+    drop_table_if_exists("airplanes");
   }
 
 protected:
@@ -38,19 +38,62 @@ using namespace matador;
 
 TEST_CASE_METHOD(SessionFixture, "Session relation test", "[session][relation]") {
   using namespace matador;
-  ses.attach<test::airplane>("airplane");
+  ses.attach<test::airplane>("airplanes");
+  ses.attach<test::flight>("flights");
   ses.create_schema();
-  ses.insert<test::airplane>(1, "Boeing", "A380");
+  auto plane = ses.insert<test::airplane>(1, "Boeing", "A380");
+  auto f = ses.insert<test::flight>(2, plane, "sully");
 
-  REQUIRE(true);
+  auto result = ses.find<test::flight>(2);
+  REQUIRE(result.is_ok());
 }
 
-TEST_CASE_METHOD(SessionFixture, "Find object with id", "[session][find]") {
+TEST_CASE_METHOD(SessionFixture, "Use session to find object with id", "[session][find]") {
   using namespace matador::test;
-  ses.attach<airplane>("airplane");
+  ses.attach<airplane>("airplanes");
   ses.create_schema();
   auto a380 = ses.insert<airplane>(1, "Boeing", "A380");
 
-  ses.find<airplane>(1);
+  auto result = ses.find<airplane>(2);
+  REQUIRE(!result.is_ok());
+  REQUIRE((result.err() == sql::session_error::FailedToFindObject));
+
+  result = ses.find<airplane>(1);
+
+  REQUIRE(result);
+  auto read_a380 = result.value();
+  REQUIRE(a380->id == read_a380->id);
+}
+
+TEST_CASE_METHOD(SessionFixture, "Use session to find all objects", "[session][find]") {
+  using namespace matador::test;
+  ses.attach<airplane>("airplanes");
+  ses.create_schema();
+
+  std::vector<std::unique_ptr<airplane>> planes;
+  planes.emplace_back(new airplane(1, "Airbus", "A380"));
+  planes.emplace_back(new airplane(2, "Boeing", "707"));
+  planes.emplace_back(new airplane(3, "Boeing", "747"));
+
+  for (auto &&plane: planes) {
+    ses.insert(plane.release());
+  }
+
+  auto result = ses.find<airplane>();
+
+  std::vector<std::tuple<int, std::string, std::string>> expected_result {
+    {1, "Airbus", "A380"},
+    {2, "Boeing", "707"},
+    {3, "Boeing", "747"}
+  };
+  REQUIRE(result);
+  auto all_planes = result.release();
+  size_t index {0};
+  for (const auto &i: all_planes) {
+    REQUIRE(i.id == std::get<0>(expected_result[index]));
+    REQUIRE(i.brand == std::get<1>(expected_result[index]));
+    REQUIRE(i.model == std::get<2>(expected_result[index]));
+    ++index;
+  }
 
 }

backends/tests/TypeTraitsTest.cpp

@@ -52,38 +52,32 @@ static const matador::utils::enum_mapper<Color> color_enum({
                                                            {Color::Brown,  "brown"}
                                                            });
 
-namespace matador::sql {
-
 template<>
-struct data_type_traits<Color, void>
+struct matador::sql::data_type_traits<Color, void>
 {
-  inline static data_type_t builtin_type(std::size_t size)
-  { return data_type_traits<std::string>::builtin_type(size); }
+    inline static data_type_t builtin_type(std::size_t size)
+    { return data_type_traits<std::string>::builtin_type(size); }
 
-  static void read_value(query_result_reader &reader, const char *id, size_t index, Color &value)
-  {
-    std::string enum_string;
-    reader.read_value(id, index, enum_string, 64);
-    auto enum_opt = color_enum.to_enum(enum_string);
-    if (enum_opt) {
-      value = enum_opt.value();
+    static void read_value(query_result_reader &reader, const char *id, size_t index, Color &value)
+    {
+        std::string enum_string;
+        reader.read_value(id, index, enum_string, 64);
+        if (const auto enum_opt = color_enum.to_enum(enum_string)) {
+            value = enum_opt.value();
+        }
     }
-  }
 
-  static any_type create_value(Color &value)
-  {
-    return color_enum.to_string(value);
-  }
+    static any_type create_value(const Color &value)
+    {
+        return color_enum.to_string(value);
+    }
 
-  static void bind_value(parameter_binder &binder, size_t index, Color &value)
-  {
-    binder.bind(index, color_enum.to_string(value));
-  }
+    static void bind_value(parameter_binder &binder, size_t index, Color &value)
+    {
+        binder.bind(index, color_enum.to_string(value));
+    }
 };
 
-}
-
-
 TEST_CASE_METHOD(TypeTraitsTestFixture, "Special handling of attributes with type traits", "[typetraits]")
 {
   schema.attach<location>("location");

demo/main.cpp

@@ -5,166 +5,291 @@
 #include "matador/sql/entity.hpp"
 
 #include "matador/utils/access.hpp"
+#include "matador/utils/enum_mapper.hpp"
 
 #include "matador/sql/query_helper.hpp"
 
 #include <iostream>
 #include <string>
 
-struct author
-{
-  unsigned long id{};
-  std::string first_name;
-  std::string last_name;
-  std::string date_of_birth;
-  unsigned short year_of_birth{};
-  bool distinguished{false};
+struct author {
+    unsigned long id{};
+    std::string first_name;
+    std::string last_name;
+    std::string date_of_birth;
+    unsigned short year_of_birth{};
+    bool distinguished{false};
 
-  template<typename Operator>
-  void process(Operator &op)
-  {
-    namespace field = matador::utils::access;
-    field::primary_key(op, "id", id);
-    field::attribute(op, "first_name", first_name, 63);
-    field::attribute(op, "last_name", last_name, 63);
-    field::attribute(op, "date_of_birth", date_of_birth, 31);
-    field::attribute(op, "year_of_birth", year_of_birth);
-    field::attribute(op, "distinguished", distinguished);
-  }
+    template<typename Operator>
+    void process( Operator& op ) {
+        namespace field = matador::utils::access;
+        field::primary_key( op, "id", id );
+        field::attribute( op, "first_name", first_name, 63 );
+        field::attribute( op, "last_name", last_name, 63 );
+        field::attribute( op, "date_of_birth", date_of_birth, 31 );
+        field::attribute( op, "year_of_birth", year_of_birth );
+        field::attribute( op, "distinguished", distinguished );
+    }
 };
 
-struct book
-{
-  unsigned long id{};
-  matador::sql::entity<author> book_author;
-  std::string title;
-  unsigned short published_in{};
+struct book {
+    unsigned long id{};
+    matador::sql::entity<author> book_author;
+    std::string title;
+    unsigned short published_in{};
 
-  template<typename Operator>
-  void process(Operator &op)
-  {
-    namespace field = matador::utils::access;
-    field::primary_key(op, "id", id);
-    field::attribute(op, "title", title, 511);
-    field::has_one(op, "author_id", book_author, matador::utils::default_foreign_attributes);
-    field::attribute(op, "published_in", published_in);
-  }
+    template<typename Operator>
+    void process( Operator& op ) {
+        namespace field = matador::utils::access;
+        field::primary_key( op, "id", id );
+        field::attribute( op, "title", title, 511 );
+        field::has_one( op, "author_id", book_author, matador::utils::default_foreign_attributes );
+        field::attribute( op, "published_in", published_in );
+    }
 };
 
-QUERY_HELPER(authors, id, first_name, last_name, date_of_birth, year_of_birth, distinguished)
-QUERY_HELPER(books, id, author_id, title, published_in)
+struct payload {
+    unsigned long id{};
 
-int main()
+    template<typename Operator>
+    void process( Operator& op ) {
+        namespace field = matador::utils::access;
+        field::primary_key( op, "id", id );
+    }
+};
+
+struct job {
+    enum class job_state {
+        Pending,
+        Running,
+        Succeeded,
+        Failed,
+        Canceled
+    };
+
+    enum class job_mode {
+        Foreground,
+        Background
+    };
+
+    unsigned long id{};
+    matador::sql::entity<payload> payload;
+    std::string type;
+    std::string description;
+    job_state state;
+    job_mode mode;
+
+    template<typename Operator>
+    void process( Operator& op ) {
+        namespace field = matador::utils::access;
+        field::primary_key( op, "id", id );
+        field::belongs_to( op, "payload", payload, matador::utils::default_foreign_attributes );
+        field::attribute( op, "type", type, 511 );
+        field::attribute( op, "description", description, 511 );
+        field::attribute( op, "state", state );
+        field::attribute( op, "mode", mode );
+    }
+};
+
+static const matador::utils::enum_mapper<job::job_state> job_state_enum({
+                                                           {job::job_state::Pending,  "Pending"},
+                                                           {job::job_state::Running,  "Running"},
+                                                           {job::job_state::Succeeded,  "Succeeded"},
+                                                           {job::job_state::Failed,  "Failed"},
+                                                           {job::job_state::Canceled,  "Canceled"}
+                                                           });
+
+static const matador::utils::enum_mapper<job::job_mode> job_mode_enum({
+                                                           {job::job_mode::Foreground,  "Foreground"},
+                                                           {job::job_mode::Background,  "Background"}
+                                                           });
+
+template<>
+struct matador::sql::data_type_traits<job::job_state, void>
 {
-  using namespace matador::sql;
-  using namespace matador;
+    inline static data_type_t builtin_type(std::size_t size)
+    { return data_type_traits<std::string>::builtin_type(size); }
 
-  const std::string env_var{"MATADOR_BACKENDS_PATH"};
+    static void read_value(query_result_reader &reader, const char *id, size_t index, job::job_state &value)
+    {
+        std::string enum_string;
+        reader.read_value(id, index, enum_string, 64);
+        if (const auto enum_opt = job_state_enum.to_enum(enum_string)) {
+            value = enum_opt.value();
+        }
+    }
 
-  std::string dns{"sqlite://demo.db"};
-  schema s("main");
-  s.attach<author>("authors");
-  s.attach<book>("books");
+    static any_type create_value(const job::job_state &value)
+    {
+        return job_state_enum.to_string(value);
+    }
 
-  connection c(dns);
-  c.open();
+    static void bind_value(parameter_binder &binder, size_t index, job::job_state &value)
+    {
+        binder.bind(index, job_state_enum.to_string(value));
+    }
+};
 
-  auto create_authors_sql = c.query(s)
-    .create()
-    .table<author>(qh::authors)
-    .execute();
+template<>
+struct matador::sql::data_type_traits<job::job_mode, void>
+{
+    inline static data_type_t builtin_type(std::size_t size)
+    { return data_type_traits<std::string>::builtin_type(size); }
 
-  c.query(s)
-    .create()
-    .table<book>(qh::books)
-    .execute();
+    static void read_value(query_result_reader &reader, const char *id, size_t index, job::job_mode &value)
+    {
+        std::string enum_string;
+        reader.read_value(id, index, enum_string, 64);
+        if (const auto enum_opt = job_mode_enum.to_enum(enum_string)) {
+            value = enum_opt.value();
+        }
+    }
 
-  std::cout << "SQL: " << create_authors_sql << "\n";
+    static any_type create_value(const job::job_mode &value)
+    {
+        return job_mode_enum.to_string(value);
+    }
 
-  author mc;
-  mc.id = 1;
-  mc.first_name = "Michael";
-  mc.last_name = "Crichton";
-  mc.date_of_birth = "19.8.1954";
-  mc.year_of_birth = 1954;
-  mc.distinguished = true;
-  auto insert_authors_sql = c.query(s)
-    .insert()
-    .into(qh::authors)
-    .values(mc)
-    .execute();
+    static void bind_value(parameter_binder &binder, size_t index, job::job_mode &value)
+    {
+        binder.bind(index, job_mode_enum.to_string(value));
+    }
+};
 
-  std::cout << "SQL: " << insert_authors_sql << "\n";
+QUERY_HELPER( authors, id, first_name, last_name, date_of_birth, year_of_birth, distinguished )
 
-  auto result = c.query(s)
-    .select(qh::authors.columns)
-    .from(qh::authors)
-    .fetch_all();
+QUERY_HELPER( books, id, author_id, title, published_in )
 
-  for (const auto &row: result) {
-    std::cout << "Author " << row.at(qh::authors.first_name) << "\n";
-  }
+QUERY_HELPER( job, id, payload, type, description, state, mode )
 
-  auto update_authors_sql = c.query(s)
-    .update(qh::authors)
-    .set({{qh::authors.first_name, "Stephen"},
-          {qh::authors.last_name,  "King"}})
-    .where(qh::authors.last_name == "Crichton")
-    .execute();
+QUERY_HELPER( payload, id )
 
-  std::cout << "SQL: " << update_authors_sql << "\n";
+QUERY_HELPER( temporary_table, id );
 
-  auto authors = c.query(s)
-    .select(qh::authors.columns)
-    .from(qh::authors)
-    .fetch_all<author>();
+int main() {
+    using namespace matador::sql;
+    using namespace matador;
 
-  for (const auto &a: authors) {
-    std::cout << "Author " << a.first_name << "\n";
-  }
+    const std::string env_var{"MATADOR_BACKENDS_PATH"};
 
-  c.query(s)
-    .insert()
-    .into(qh::books)
-    .values({2, "It", mc.id, 1980})
-    .execute();
+    std::string dns{"sqlite://demo.db"};
+    schema s( "main" );
+    s.attach<author>( "authors" );
+    s.attach<book>( "books" );
 
-  c.query(s)
-    .insert()
-    .into(qh::books)
-    .values({3, "Misery", mc.id, 1984})
-    .execute();
+    connection c( dns );
+    c.open();
+    s.create( c );
 
-  auto select_books_sql = c.query(s)
-    .select(qh::books.columns, {qh::authors.last_name})
-    .from(qh::books)
-    .join_left(qh::authors)
-    .on(qh::books.author_id == qh::authors.id)
-    .where(qh::books.published_in < 2008 && qh::authors.last_name == "King")
-    .group_by(qh::books.published_in)
-    .order_by(qh::books.title).asc()
-    .limit(5)
-    .offset(2)
-    .fetch_all();
+    auto create_authors_sql = c.query( s )
+                               .create()
+                               .table<author>( qh::authors )
+                               .execute();
 
-  for (const auto &r: select_books_sql) {
-    std::cout << "R: " << r.at(qh::books.title) << ", " << r.at(qh::authors.last_name) << "\n";
-  }
-  // SELECT book.title, book.id, book.author_id, book.published_in, author.name
-  // FROM book
-  // INNER JOIN author ON book.author_id = author.id
-  // WHERE book.published_in < 2008 AND author.name = "Michael Crichton"
-  // ORDER BY "book.title" ASC
-  // OFFSET 2 LIMIT 5
+    c.query( s )
+     .create()
+     .table<book>( qh::books )
+     .execute();
 
-  c.query(s).drop().table(qh::books).execute();
+    std::cout << "SQL: " << create_authors_sql << "\n";
 
-  auto drop_authors_sql = c.query(s)
-    .drop()
-    .table(qh::authors)
-    .execute();
+    author mc;
+    mc.id = 1;
+    mc.first_name = "Michael";
+    mc.last_name = "Crichton";
+    mc.date_of_birth = "19.8.1954";
+    mc.year_of_birth = 1954;
+    mc.distinguished = true;
+    auto insert_authors_sql = c.query( s )
+                               .insert()
+                               .into( qh::authors )
+                               .values( mc )
+                               .execute();
 
-  std::cout << "SQL: " << drop_authors_sql << "\n";
+    std::cout << "SQL: " << insert_authors_sql << "\n";
 
-  return 0;
+    auto result = c.query( s )
+                   .select( qh::authors.columns )
+                   .from( qh::authors )
+                   .fetch_all();
+
+    for (const auto& row: result) { std::cout << "Author " << row.at( qh::authors.first_name ) << "\n"; }
+
+    auto update_authors_sql = c.query( s )
+                               .update( qh::authors )
+                               .set( {{qh::authors.first_name, "Stephen"},
+                                      {qh::authors.last_name, "King"}} )
+                               .where( qh::authors.last_name == "Crichton" )
+                               .execute();
+
+    std::cout << "SQL: " << update_authors_sql << "\n";
+
+    auto authors = c.query( s )
+                    .select( qh::authors.columns )
+                    .from( qh::authors )
+                    .fetch_all<author>();
+
+    for (const auto& a: authors) { std::cout << "Author " << a.first_name << "\n"; }
+
+    c.query( s )
+     .insert()
+     .into( qh::books )
+     .values( {2, "It", mc.id, 1980} )
+     .execute();
+
+    c.query( s )
+     .insert()
+     .into( qh::books )
+     .values( {3, "Misery", mc.id, 1984} )
+     .execute();
+
+    auto select_books_sql = c.query( s )
+                             .select( qh::books.columns, {qh::authors.last_name} )
+                             .from( qh::books )
+                             .join_left( qh::authors )
+                             .on( qh::books.author_id == qh::authors.id )
+                             .where( qh::books.published_in < 2008 && qh::authors.last_name == "King" )
+                             .group_by( qh::books.published_in )
+                             .order_by( qh::books.title ).asc()
+                             .limit( 5 )
+                             .offset( 2 )
+                             .fetch_all();
+
+    for (const auto& r: select_books_sql) { std::cout << "R: " << r.at( qh::books.title ) << ", " << r.at( qh::authors.last_name ) << "\n"; }
+    // SELECT book.title, book.id, book.author_id, book.published_in, author.name
+    // FROM book
+    // INNER JOIN author ON book.author_id = author.id
+    // WHERE book.published_in < 2008 AND author.name = "Michael Crichton"
+    // ORDER BY "book.title" ASC
+    // OFFSET 2 LIMIT 5
+
+    c.query( s ).drop().table( qh::books ).execute();
+
+    auto drop_authors_sql = c.query( s )
+                             .drop()
+                             .table( qh::authors )
+                             .execute();
+
+    std::cout << "SQL: " << drop_authors_sql << "\n";
+
+    auto res = c.query( s )
+                .select( {qh::payload.id} )
+                .from( qh::payload )
+                .join_left( qh::job )
+                .on( qh::job.payload == qh::payload.id )
+                .where(
+                        in( qh::payload.id, c.query( s )
+                                             .select( {qh::job.state} )
+                                             .from( qh::job )
+                                             .where( qh::job.state == job::job_state::Running )
+                                ) &&
+                        in( qh::payload.id, c.query( s )
+                                             .select( {qh::temporary_table.id} )
+                                             .from( qh::temporary_table ) )
+                        )
+                .build();
+    // .fetch_value<unsigned long>();
+    std::cout << "SQL: " << res.sql << "\n";
+
+    return 0;
 }

include/matador/query/column.hpp

@@ -1,25 +0,0 @@
-#ifndef QUERY_COLUMN_HPP
-#define QUERY_COLUMN_HPP
-
-#include <string>
-
-namespace matador::query {
-
-class column {
-public:
-  column(const char *name); // NOLINT(*-explicit-constructor)
-  column(std::string name); // NOLINT(*-explicit-constructor)
-  column(std::string table_name, std::string name, std::string as);
-
-  column& as(std::string a);
-
-  std::string table;
-  std::string name;
-  std::string alias;
-};
-
-column operator "" _col(const char *name, size_t len);
-
-}
-
-#endif //QUERY_COLUMN_HPP

include/matador/query/query.hpp

@@ -1,22 +0,0 @@
-#ifndef QUERY_QUERY_HPP
-#define QUERY_QUERY_HPP
-
-#include "matador/query/query_parts.hpp"
-
-namespace matador::query {
-
-class connection;
-class query
-{
-public:
-  explicit query(connection &c);
-
-  query& select(const std::vector<column> &columns);
-  query& from(const std::string &table, const std::string &as = "");
-
-private:
-  connection &connection_;
-};
-
-}
-#endif //QUERY_QUERY_HPP

include/matador/query/query_builder.hpp

@@ -1,24 +0,0 @@
-#ifndef QUERY_QUERY_BUILDER_HPP
-#define QUERY_QUERY_BUILDER_HPP
-
-#include "matador/query/column.hpp"
-#include "matador/query/query_intermediates.hpp"
-
-#include "matador/sql/schema.hpp"
-
-#include <memory>
-
-namespace matador::query {
-
-class query_builder
-{
-public:
-  explicit query_builder(const std::shared_ptr<sql::schema> &scm);
-  query_select_intermediate select(std::initializer_list<column> columns);
-
-private:
-  std::shared_ptr<sql::schema> schema_;
-};
-
-}
-#endif //QUERY_QUERY_BUILDER_HPP

include/matador/query/query_compiler.hpp

@@ -1,37 +0,0 @@
-#ifndef QUERY_QUERY_COMPILER_HPP
-#define QUERY_QUERY_COMPILER_HPP
-
-#include "matador/query/query_part_visitor.hpp"
-#include "matador/query/query_parts.hpp"
-
-
-#include <typeindex>
-#include <string>
-
-namespace matador::sql {
-class dialect;
-}
-
-namespace matador::query {
-
-struct query_data;
-
-class query_compiler : public query_part_visitor
-{
-public:
-  explicit query_compiler(sql::dialect& d);
-
-  std::string compile(const query_data *data);
-
-private:
-  void visit(query_select_part &select_part) override;
-  void visit(query_from_part &from_part) override;
-
-private:
-  sql::dialect &dialect_;
-  std::string sql_;
-};
-
-}
-
-#endif //QUERY_QUERY_COMPILER_HPP

include/matador/query/query_data.hpp

@@ -1,24 +0,0 @@
-#ifndef QUERY_QUERY_DATA_HPP
-#define QUERY_QUERY_DATA_HPP
-
-#include "matador/query/query_parts.hpp"
-#include "matador/query/sql_commands.hpp"
-#include "matador/query/table.hpp"
-
-#include <memory>
-#include <vector>
-
-namespace matador::query {
-
-struct query_data
-{
-  SqlCommands command;
-  std::vector<std::unique_ptr<query_part>> parts;
-  std::vector<column> columns;
-  std::unordered_map<std::type_index, table_data> table_map_by_index;
-  using table_data_ref = std::reference_wrapper<table_data>;
-  std::unordered_map<std::string, table_data_ref> table_map_by_name;
-};
-
-}
-#endif //QUERY_QUERY_DATA_HPP

include/matador/query/query_intermediates.hpp

@@ -1,132 +0,0 @@
-#ifndef QUERY_QUERY_INTERMEDIATES_HPP
-#define QUERY_QUERY_INTERMEDIATES_HPP
-
-#include "matador/sql/schema.hpp"
-
-#include <string>
-#include <memory>
-
-namespace matador::sql {
-class basic_condition;
-}
-namespace matador::query {
-
-class query_from_intermediate;
-class query_join_intermediate;
-class query_on_intermediate;
-class query_where_intermediate;
-class query_group_by_intermediate;
-class query_order_by_intermediate;
-class query_order_direction_intermediate;
-class query_offset_intermediate;
-class query_limit_intermediate;
-
-class query_intermediate
-{
-public:
-  explicit query_intermediate(const std::shared_ptr<sql::schema> &scm);
-
-protected:
-  sql::schema& schema() const;
-
-private:
-  std::shared_ptr<sql::schema> schema_;
-};
-
-class query_select_finish_intermediate : public query_intermediate
-{
-public:
-  using query_intermediate::query_intermediate;
-
-  void compile();
-};
-
-class query_select_intermediate : public query_intermediate
-{
-public:
-  using query_intermediate::query_intermediate;
-
-  query_from_intermediate from(const std::string &table, const std::string &as = "");
-};
-
-class query_from_intermediate : public query_select_finish_intermediate
-{
-public:
-  using query_select_finish_intermediate::query_intermediate;
-
-  query_join_intermediate join(const std::string &join_table_name, const std::string &as);
-  query_where_intermediate where(const sql::basic_condition &cond);
-  query_group_by_intermediate group_by(const std::string &name);
-  query_order_by_intermediate order_by(const std::string &name);
-};
-
-class query_join_intermediate : public query_intermediate
-{
-public:
-  using query_intermediate::query_intermediate;
-
-  query_on_intermediate on(const std::string &column, const std::string &join_column);
-};
-
-class query_on_intermediate : public query_select_finish_intermediate
-{
-public:
-  using query_select_finish_intermediate::query_intermediate;
-
-  query_join_intermediate join(const std::string &join_table_name, const std::string &as);
-  query_where_intermediate where(const sql::basic_condition &cond);
-  query_group_by_intermediate group_by(const std::string &name);
-  query_order_by_intermediate order_by(const std::string &name);
-};
-
-class query_where_intermediate : public query_select_finish_intermediate
-{
-public:
-  using query_select_finish_intermediate::query_intermediate;
-
-  query_group_by_intermediate group_by(const std::string &name);
-  query_order_by_intermediate order_by(const std::string &name);
-};
-
-class query_group_by_intermediate : public query_select_finish_intermediate
-{
-public:
-  using query_select_finish_intermediate::query_intermediate;
-
-  query_order_by_intermediate order_by(const std::string &name);
-};
-
-class query_order_by_intermediate : public query_intermediate
-{
-public:
-  using query_intermediate::query_intermediate;
-
-  query_order_direction_intermediate asc();
-  query_order_direction_intermediate desc();
-};
-
-class query_order_direction_intermediate : public query_select_finish_intermediate
-{
-public:
-  using query_select_finish_intermediate::query_intermediate;
-
-  query_offset_intermediate offset(size_t offset);
-  query_limit_intermediate limit(size_t limit);
-};
-
-class query_offset_intermediate : public query_intermediate
-{
-public:
-  using query_intermediate::query_intermediate;
-
-  query_limit_intermediate limit(size_t limit);
-};
-
-class query_limit_intermediate : public query_select_finish_intermediate
-{
-public:
-  using query_select_finish_intermediate::query_intermediate;
-};
-
-}
-#endif //QUERY_QUERY_INTERMEDIATES_HPP

include/matador/query/query_part_visitor.hpp

@@ -1,19 +0,0 @@
-#ifndef QUERY_QUERY_PART_VISITOR_HPP
-#define QUERY_QUERY_PART_VISITOR_HPP
-
-namespace matador::query {
-
-class query_select_part;
-class query_from_part;
-
-class query_part_visitor
-{
-public:
-  virtual ~query_part_visitor() = default;
-
-  virtual void visit(query_select_part &select_part) = 0;
-  virtual void visit(query_from_part &from_part) = 0;
-};
-
-}
-#endif //QUERY_QUERY_PART_VISITOR_HPP

include/matador/query/query_parts.hpp

@@ -1,55 +0,0 @@
-#ifndef QUERY_QUERY_PARTS_HPP
-#define QUERY_QUERY_PARTS_HPP
-
-#include "matador/query/query_part_visitor.hpp"
-#include "matador/query/column.hpp"
-
-#include "matador/sql/dialect.hpp"
-
-namespace matador::query {
-
-class query_part
-{
-protected:
-  explicit query_part(sql::dialect::token_t token);
-
-public:
-  virtual ~query_part() = default;
-  virtual void accept(query_part_visitor &visitor) = 0;
-
-protected:
-  sql::dialect::token_t token_;
-};
-
-/**
- * Represents the SQL SELECT part
- */
-class query_select_part : public query_part
-{
-public:
-  explicit query_select_part(std::vector<column> columns);
-  void accept(query_part_visitor &visitor) override;
-
-private:
-  std::vector<column> columns_;
-};
-
-/**
- * Represents the SQL FROM part
- */
-class query_from_part : public query_part
-{
-public:
-  explicit query_from_part(std::string table_name);
-  query_from_part(std::string table_name, std::string as);
-
-private:
-  void accept(query_part_visitor &visitor) override;
-
-private:
-  std::string table_;
-  std::string alias_;
-};
-
-}
-#endif //QUERY_QUERY_PARTS_HPP

include/matador/query/sql_commands.hpp

@@ -1,31 +0,0 @@
-#ifndef QUERY_SQL_COMMANDS_HPP
-#define QUERY_SQL_COMMANDS_HPP
-
-#include "matador/utils/enum_mapper.hpp"
-
-namespace matador::query {
-
-enum class SqlCommands
-{
-  UNKNOWN,   /**< Unknown query command */
-  CREATE,    /**< Create query command */
-  DROP,      /**< Drop query command */
-  SELECT,    /**< Select query command */
-  INSERT,    /**< Insert query command */
-  UPDATE,    /**< Update query command */
-  REMOVE     /**< Remove query command */
-};
-
-static const utils::enum_mapper<SqlCommands> sql_command_enum({
-                                                                {SqlCommands::UNKNOWN, "unknown"},
-                                                                {SqlCommands::CREATE,  "create"},
-                                                                {SqlCommands::DROP,    "drop"},
-                                                                {SqlCommands::SELECT,  "select"},
-                                                                {SqlCommands::INSERT,  "insert"},
-                                                                {SqlCommands::UPDATE,  "update"},
-                                                                {SqlCommands::REMOVE,  "delete"}
-                                                              });
-
-}
-
-#endif //QUERY_SQL_COMMANDS_HPP

include/matador/query/table.hpp

@@ -1,16 +0,0 @@
-#ifndef QUERY_TABLE_HPP
-#define QUERY_TABLE_HPP
-
-#include <typeindex>
-#include <string>
-
-namespace matador::query {
-
-struct table_data
-{
-  std::type_index index;
-  std::string name;
-};
-
-}
-#endif //QUERY_TABLE_HPP

include/matador/sql/basic_condition.hpp

@@ -33,7 +33,7 @@ public:
     LIKE
   };
 
-  virtual std::string evaluate(dialect &dialect, query_context &query) const = 0;
+  virtual std::string evaluate(const dialect &dialect, query_context &query) const = 0;
 
   static std::unordered_map<operand_t, std::string> operands;
 };

include/matador/sql/column.hpp

@@ -21,11 +21,11 @@ struct column
   column(const char *name); // NOLINT(*-explicit-constructor)
   column(std::string name); // NOLINT(*-explicit-constructor)
   column(sql_function_t func, std::string name); // NOLINT(*-explicit-constructor)
-  column(std::string table_name, std::string name, std::string as);
-  column(std::string table_name, const char* name, std::string as);
-  column(table &t, const char* name, std::string as);
+  column(std::string table_name, std::string name, std::string as = "");
+  column(std::string table_name, const char* name, std::string as = "");
+  column(struct table &t, const char* name, std::string as = "");
 
-  bool equals(const column &x) const;
+  [[nodiscard]] bool equals(const column &x) const;
 
   column& as(std::string a);
 

include/matador/sql/column_definition.hpp

@@ -53,6 +53,15 @@ public:
   [[nodiscard]] const std::string& ref_table() const;
   [[nodiscard]] const std::string& ref_column() const;
 
+  [[nodiscard]] bool is_integer() const;
+  [[nodiscard]] bool is_floating_point() const;
+  [[nodiscard]] bool is_bool() const;
+  [[nodiscard]] bool is_string() const;
+  [[nodiscard]] bool is_varchar() const;
+  [[nodiscard]] bool is_blob() const;
+  [[nodiscard]] bool is_null() const;
+  [[nodiscard]] bool is_unknown() const;
+
   void type(data_type_t type);
 
   template< typename Type >

include/matador/sql/column_definition_generator.hpp

@@ -42,9 +42,11 @@ public:
   template<class Pointer>
   void on_has_one(const char * /*id*/, Pointer &/*x*/, const utils::foreign_attributes &/*attr*/) {}
   template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const char *, const char *, const utils::foreign_attributes &/*attr*/) {}
+  void on_has_many(ContainerType &, const char *, const utils::foreign_attributes &/*attr*/) {}
   template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const utils::foreign_attributes &/*attr*/) {}
+  void on_has_many_to_many(const char *id, ContainerType &c, const char *join_column, const char *inverse_join_column, const utils::foreign_attributes &/*attr*/) {}
+  template<class ContainerType>
+  void on_has_many_to_many(const char *id, ContainerType &c, const utils::foreign_attributes &/*attr*/) {}
 
 private:
   data_type_t type_{};
@@ -92,9 +94,11 @@ public:
     columns_.push_back(fk_column_generator_.generate(id, *x, ref_table, ref_column));
   }
   template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const char *, const char *, const utils::foreign_attributes &/*attr*/) {}
+  void on_has_many(ContainerType &, const char *, const utils::foreign_attributes &/*attr*/) {}
   template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const utils::foreign_attributes &/*attr*/) {}
+  void on_has_many_to_many(const char *id, ContainerType &c, const char *join_column, const char *inverse_join_column, const utils::foreign_attributes &/*attr*/) {}
+  template<class ContainerType>
+  void on_has_many_to_many(const char *id, ContainerType &c, const utils::foreign_attributes &/*attr*/) {}
 
 private:
   std::pair<std::string, std::string> determine_foreign_ref(const std::type_index &ti);

include/matador/sql/column_generator.hpp

@@ -86,7 +86,7 @@ public:
     }
   }
   template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const char *, const char *, const utils::foreign_attributes &attr)
+  void on_has_many(ContainerType &, const char *, const utils::foreign_attributes &attr)
   {
     if (attr.fetch() == utils::fetch_type::LAZY || force_lazy_) {
       return;
@@ -101,10 +101,15 @@ public:
     matador::utils::access::process(*this, obj);
     table_name_stack_.pop();
   }
+
   template<class ContainerType>
-  void on_has_many(const char *id, ContainerType &c, const utils::foreign_attributes &attr)
+  void on_has_many_to_many(const char *id, ContainerType &c, const char *join_column, const char *inverse_join_column, const utils::foreign_attributes &attr)
+  {
+  }
+
+  template<class ContainerType>
+  void on_has_many_to_many(const char *id, ContainerType &c, const utils::foreign_attributes &attr)
   {
-    on_has_many(id, c, "", "", attr);
   }
 
 private:

include/matador/sql/condition.hpp

@@ -1,6 +1,8 @@
 #ifndef QUERY_CONDITION_HPP
 #define QUERY_CONDITION_HPP
 
+#include "matador/sql/any_type_to_string_visitor.hpp"
+#include "matador/sql/query_result.hpp"
 #include "matador/sql/basic_condition.hpp"
 #include "matador/sql/dialect.hpp"
 #include "matador/sql/placeholder.hpp"
@@ -26,6 +28,8 @@ namespace matador::sql {
 
 /// @cond MATADOR_DEV
 
+class query_select;
+
 template<class L, class R, class Enabled = void>
 class condition;
 
@@ -37,12 +41,13 @@ public:
 
     placeholder value;
 
-    std::string evaluate(dialect &d, query_context &query) const override;
+    std::string evaluate(const dialect &d, query_context &query) const override;
 };
 
 template<class T>
 class condition<column, T, typename std::enable_if<
   std::is_scalar<T>::value &&
+  !std::is_enum<T>::value &&
   !std::is_same<std::string, T>::value &&
   !std::is_same<const char*, T>::value>::type> : public basic_column_condition
 {
@@ -54,7 +59,7 @@ public:
 
   T value;
 
-  std::string evaluate(dialect &d, query_context &query) const override
+  std::string evaluate(const dialect &d, query_context &query) const override
   {
     query.bind_vars.emplace_back(field_.name);
     return d.prepare_identifier(field_) + " " + operand + " " + std::to_string(value);
@@ -74,7 +79,7 @@ public:
 
   T value;
 
-  std::string evaluate(dialect &d, query_context &query) const override
+  std::string evaluate(const dialect &d, query_context &query) const override
   {
     query.bind_vars.emplace_back(field_.name);
     return d.prepare_identifier(field_) + " " + operand + " '" + value + "'";
@@ -82,10 +87,32 @@ public:
 };
 
 template<class T>
-class condition<T, column, typename std::enable_if<
-  std::is_scalar<T>::value &&
-  !std::is_same<std::string, T>::value &&
-  !std::is_same<const char*, T>::value>::type> : public basic_column_condition
+class condition<column, T, std::enable_if_t<std::is_enum_v<T>>> final : public basic_column_condition
+{
+public:
+  condition(const column &fld, basic_condition::operand_t op, T val)
+    : basic_column_condition(fld, op)
+    , value(val)
+  { }
+
+  T value;
+
+  std::string evaluate(const dialect &d, query_context &query) const override
+  {
+    auto at = data_type_traits<T>::create_value(value);
+    any_type_to_string_visitor value_to_string(d, query);
+    std::visit(value_to_string, at);
+    return value_to_string.result + " " + operand + " " + d.prepare_identifier(field_);
+  }
+
+};
+
+
+template<class T>
+class condition<T, column, std::enable_if_t<
+  std::is_scalar_v<T> &&
+  !std::is_same_v<std::string, T> &&
+  !std::is_same_v<const char*, T>>> final : public basic_column_condition
 {
 public:
   condition(T val, basic_condition::operand_t op, const column &fld)
@@ -95,7 +122,7 @@ public:
 
   T value;
 
-  std::string evaluate(dialect &d, query_context &query) const override
+  std::string evaluate(const dialect &d, query_context &query) const override
   {
     return std::to_string(value) + " " + operand + " " + d.prepare_identifier(field_);
   }
@@ -114,7 +141,7 @@ public:
 
   T value;
 
-  std::string evaluate(dialect &d, query_context &query) const override
+  std::string evaluate(const dialect &d, query_context &query) const override
   {
     return "'" + std::to_string(value) + "' " + operand + " " + d.prepare_identifier(field_);
   }
@@ -156,7 +183,7 @@ public:
    * @param d The d used to evaluate
    * @return A condition IN part of the query
    */
-  std::string evaluate(dialect &d, query_context &query) const override {
+  std::string evaluate(const dialect &d, query_context &query) const override {
     auto count = size();
     for (size_t i = 0; i < count; ++i) {
       query.bind_vars.emplace_back(field_.name);
@@ -215,7 +242,7 @@ public:
    * @param op Operand of the condition
    * @param q The query to be evaluated to the IN arguments
    */
-  condition(column col, basic_condition::operand_t op, query_context &q);
+  condition(column col, basic_condition::operand_t op, const query_context &q);
 
   /**
    * @brief Evaluates the condition
@@ -226,10 +253,10 @@ public:
    * @param d The d used to evaluate
    * @return A condition IN part of the query
    */
-  std::string evaluate(dialect &d, query_context &query) const override;
+  std::string evaluate(const dialect &d, query_context &query) const override;
 
 private:
-  query_context &query_;
+  query_context query_;
 };
 
 /**
@@ -261,7 +288,7 @@ public:
    * @param d The d used to evaluate
    * @return A condition BETWEEN part of the query
    */
-  std::string evaluate(dialect &d, query_context &query) const override {
+  std::string evaluate(const dialect &d, query_context &query) const override {
     query.bind_vars.emplace_back(field_.name);
     query.bind_vars.emplace_back(field_.name);
     return d.prepare_identifier(field_) + " BETWEEN " + std::to_string(range_.first) + " AND " + std::to_string(range_.second);
@@ -303,7 +330,7 @@ public:
    * @param d The d used to evaluate
    * @return The evaluated string based on the compile type
    */
-  std::string evaluate(dialect &d, query_context &query) const override
+  std::string evaluate(const dialect &d, query_context &query) const override
   {
     // ensure the numbering order for host vars
     auto cl = left.evaluate(d, query);
@@ -346,7 +373,7 @@ public:
    * @param d The d used to evaluate
    * @return The evaluated string based on the compile type
    */
-  std::string evaluate(dialect &d, query_context &query) const override
+  std::string evaluate(const dialect &d, query_context &query) const override
   {
     return operand + " (" + cond.evaluate(d) + ")";
   }
@@ -369,7 +396,7 @@ public:
    * @param d The d used to evaluate
    * @return The evaluated string based on the compile type
    */
-  std::string evaluate(dialect &d, query_context &query) const override
+  std::string evaluate(const dialect &d, query_context &query) const override
   {
     return d.prepare_identifier(field_) + " " + operand + " " + d.prepare_identifier(other_column_);
   }
@@ -415,7 +442,8 @@ condition<column, std::initializer_list<V>> in(const column &col, std::initializ
  * @param q The query to be executes as sub select
  * @return The condition object
  */
-condition<column, query_context> in(const column &col, query_context &&q);
+condition<column, query_context> in(const column &col, const query_context &q);
+condition<column, query_context> in(const column &col, const query_select &q);
 
 /**
  * @brief Creates a between condition.

include/matador/sql/connection_pool.hpp

@@ -7,6 +7,7 @@
 #include <mutex>
 #include <string>
 #include <optional>
+#include <condition_variable>
 #include <thread>
 #include <unordered_map>
 
@@ -86,19 +87,21 @@ public:
   }
 
   connection_pointer acquire() {
+    std::unique_lock<std::mutex> lock(mutex_);
+    while (idle_connections_.empty()) {
+      cv.wait(lock);
+    }
+
+    return get_next_connection();
+  }
+
+  connection_pointer try_acquire() {
     std::unique_lock<std::mutex> lock(mutex_);
     if (idle_connections_.empty()) {
       return {nullptr, this};
     }
 
-    pointer next_connection{nullptr};
-    for (auto &item : idle_connections_) {
-      next_connection = item.second;
-      auto node = idle_connections_.extract(item.first);
-      inuse_connections_.insert(std::move(node));
-      break;
-    }
-    return {next_connection, this};
+    return get_next_connection();
   }
 
   connection_pointer acquire(size_t id) {
@@ -151,8 +154,21 @@ public:
   const connection_info &info() const {
     return info_;
   }
+
+private:
+  connection_pointer get_next_connection() {
+    pointer next_connection{nullptr};
+    for (auto &item : idle_connections_) {
+      next_connection = item.second;
+      auto node = idle_connections_.extract(item.first);
+      inuse_connections_.insert(std::move(node));
+      break;
+    }
+    return {next_connection, this};
+  }
 private:
   mutable std::mutex mutex_;
+  std::condition_variable cv;
   std::vector<IdConnection<Connection>> connection_repo_;
   using pointer = IdConnection<Connection>*;
   using connection_map = std::unordered_map<size_t, pointer>;

include/matador/sql/data_type_traits.hpp

@@ -53,13 +53,22 @@ template < class Type, class Enable = void >
 struct data_type_traits;
 
 /// @cond MATADOR_DEV
+template <> struct data_type_traits<nullptr_t, void>
+{
+  inline static data_type_t builtin_type(std::size_t /*size*/) { return data_type_t::type_null; }
+  static void read_value(query_result_reader &reader, const char *id, size_t index, nullptr_t &/*value*/);
+  static void bind_value(parameter_binder &binder, size_t index, nullptr_t &/*value*/);
+  static void bind_result_value(result_parameter_binder &binder, size_t index, nullptr_t &/*value*/);
+  inline static any_type create_value(const char &value) { return value; }
+};
+
 template <> struct data_type_traits<char, void>
 {
   inline static data_type_t builtin_type(std::size_t /*size*/) { return data_type_t::type_char; }
   static void read_value(query_result_reader &reader, const char *id, size_t index, char &value);
   static void bind_value(parameter_binder &binder, size_t index, char &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, char &value);
-  inline static any_type create_value(char &value) { return value; }
+  inline static any_type create_value(const char &value) { return value; }
 };
 
 template <> struct data_type_traits<short, void>
@@ -68,7 +77,7 @@ template <> struct data_type_traits<short, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, short &value);
   static void bind_value(parameter_binder &binder, size_t index, short &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, short &value);
-  inline static any_type create_value(short &value) { return value; }
+  inline static any_type create_value(const short &value) { return value; }
 };
 
 template <> struct data_type_traits<int, void>
@@ -77,7 +86,7 @@ template <> struct data_type_traits<int, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, int &value);
   static void bind_value(parameter_binder &binder, size_t index, int &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, int &value);
-  inline static any_type create_value(int &value) { return value; }
+  inline static any_type create_value(const int &value) { return value; }
 };
 
 template <> struct data_type_traits<long, void>
@@ -86,7 +95,7 @@ template <> struct data_type_traits<long, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, long &value);
   static void bind_value(parameter_binder &binder, size_t index, long &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, long &value);
-  inline static any_type create_value(long &value) { return value; }
+  inline static any_type create_value(const long &value) { return value; }
 };
 
 template <> struct data_type_traits<long long, void>
@@ -95,7 +104,7 @@ template <> struct data_type_traits<long long, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, long long &value);
   static void bind_value(parameter_binder &binder, size_t index, long long &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, long long &value);
-  inline static any_type create_value(long long &value) { return value; }
+  inline static any_type create_value(const long long &value) { return value; }
 };
 
 template <> struct data_type_traits<unsigned char, void>
@@ -104,7 +113,7 @@ template <> struct data_type_traits<unsigned char, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, unsigned char &value);
   static void bind_value(parameter_binder &binder, size_t index, unsigned char &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, unsigned char &value);
-  inline static any_type create_value(unsigned char &value) { return value; }
+  inline static any_type create_value(const unsigned char &value) { return value; }
 };
 
 template <> struct data_type_traits<unsigned short, void>
@@ -113,7 +122,7 @@ template <> struct data_type_traits<unsigned short, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, unsigned short &value);
   static void bind_value(parameter_binder &binder, size_t index, unsigned short &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, unsigned short &value);
-  inline static any_type create_value(unsigned short &value) { return value; }
+  inline static any_type create_value(const unsigned short &value) { return value; }
 };
 
 template <> struct data_type_traits<unsigned int, void>
@@ -122,7 +131,7 @@ template <> struct data_type_traits<unsigned int, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, unsigned int &value);
   static void bind_value(parameter_binder &binder, size_t index, unsigned int &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, unsigned int &value);
-  inline static any_type create_value(unsigned int &value) { return value; }
+  inline static any_type create_value(const unsigned int &value) { return value; }
 };
 
 template <> struct data_type_traits<unsigned long, void>
@@ -131,7 +140,7 @@ template <> struct data_type_traits<unsigned long, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, unsigned long &value);
   static void bind_value(parameter_binder &binder, size_t index, unsigned long &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, unsigned long &value);
-  inline static any_type create_value(unsigned long &value) { return value; }
+  inline static any_type create_value(const unsigned long &value) { return value; }
 };
 
 template <> struct data_type_traits<unsigned long long, void>
@@ -140,7 +149,7 @@ template <> struct data_type_traits<unsigned long long, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, unsigned long long &value);
   static void bind_value(parameter_binder &binder, size_t index, unsigned long long &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, unsigned long long &value);
-  inline static any_type create_value(unsigned long long &value) { return value; }
+  inline static any_type create_value(const unsigned long long &value) { return value; }
 };
 
 template <> struct data_type_traits<bool, void>
@@ -149,7 +158,7 @@ template <> struct data_type_traits<bool, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, bool &value);
   static void bind_value(parameter_binder &binder, size_t index, bool &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, bool &value);
-  inline static any_type create_value(bool &value) { return value; }
+  inline static any_type create_value(const bool &value) { return value; }
 };
 
 template <> struct data_type_traits<float, void>
@@ -158,7 +167,7 @@ template <> struct data_type_traits<float, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, float &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, float &value);
   static void bind_value(parameter_binder &binder, size_t index, float &value);
-  inline static any_type create_value(float &value) { return value; }
+  inline static any_type create_value(const float &value) { return value; }
 };
 
 template <> struct data_type_traits<double, void>
@@ -167,7 +176,7 @@ template <> struct data_type_traits<double, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, double &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, double &value);
   static void bind_value(parameter_binder &binder, size_t index, double &value);
-  inline static any_type create_value(double &value) { return value; }
+  inline static any_type create_value(const double &value) { return value; }
 };
 
 template <> struct data_type_traits<const char*, void>
@@ -194,16 +203,16 @@ template <> struct data_type_traits<std::string, void>
   static void read_value(query_result_reader &reader, const char *id, size_t index, std::string &value, size_t size);
   static void bind_value(parameter_binder &binder, size_t index, std::string &value, size_t size = 0);
   static void bind_result_value(result_parameter_binder &binder, size_t index, std::string &value, size_t size = 0);
-  inline static any_type create_value(std::string &value) { return value; }
+  inline static any_type create_value(const std::string &value) { return value; }
 };
 
 template <> struct data_type_traits<utils::blob, void>
 {
-  inline static data_type_t builtin_type(std::size_t size) { return data_type_t::type_blob; }
+  inline static data_type_t builtin_type(std::size_t /*size*/) { return data_type_t::type_blob; }
   static void read_value(query_result_reader &reader, const char *id, size_t index, utils::blob &value);
   static void bind_value(parameter_binder &binder, size_t index, utils::blob &value);
   static void bind_result_value(result_parameter_binder &binder, size_t index, utils::blob &value);
-  inline static any_type create_value(utils::blob &value) { return value; }
+  inline static any_type create_value(const utils::blob &value) { return value; }
 };
 
 //template <> struct data_type_traits<matador::date>
@@ -223,7 +232,7 @@ template <> struct data_type_traits<utils::blob, void>
 //};
 
 template < typename EnumType >
-struct data_type_traits<EnumType, typename std::enable_if<std::is_enum<EnumType>::value>::type>
+struct data_type_traits<EnumType, std::enable_if_t<std::is_enum_v<EnumType>>>
 {
   inline static data_type_t builtin_type(std::size_t /*size*/) { return data_type_t::type_int; }
   static void read_value(query_result_reader &reader, const char *id, size_t index, EnumType &value)
@@ -232,13 +241,15 @@ struct data_type_traits<EnumType, typename std::enable_if<std::is_enum<EnumType>
   }
   static void bind_value(parameter_binder &binder, size_t index, EnumType &value)
   {
-    data_type_traits<int>::bind_value(binder, index, (int&)value);
+    data_type_traits<int>::bind_value(binder, index, static_cast<int&>(value));
   }
   static void bind_result_value(result_parameter_binder &binder, size_t index, EnumType &value)
   {
-    data_type_traits<int>::bind_result_value(binder, index, (int&)value);
+    data_type_traits<int>::bind_result_value(binder, index, static_cast<int&>(value));
+  }
+  static any_type create_value(const EnumType &value) {
+      return static_cast<int>(value);
   }
-  inline static any_type create_value(EnumType &value) { return (int)value; }
 };
 /// @endcond
 

include/matador/sql/entity_query_builder.hpp

@@ -2,112 +2,282 @@
 #define QUERY_ENTITY_QUERY_BUILDER_HPP
 
 #include "matador/sql/connection.hpp"
+#include "matador/sql/condition.hpp"
 #include "matador/sql/query_context.hpp"
 #include "matador/sql/query.hpp"
 #include "matador/sql/query_intermediates.hpp"
+#include "matador/sql/value.hpp"
+
+#include "matador/utils/result.hpp"
 
 #include <iostream>
 
 namespace matador::sql {
 
-template < typename PrimaryKeyType >
+struct join_columns
+{
+  std::string join_column;
+  std::string inverse_join_column;
+};
+
+class join_column_collector
+{
+public:
+  template<class Type>
+  join_columns collect()
+  {
+    join_columns_ = {};
+    Type obj;
+
+    matador::utils::access::process(*this, obj);
+
+    return join_columns_;
+  }
+  template < class V >
+  void on_primary_key(const char * /*id*/, V &, typename std::enable_if<std::is_integral<V>::value && !std::is_same<bool, V>::value>::type* = 0) {}
+  void on_primary_key(const char * /*id*/, std::string &, size_t) {}
+  void on_revision(const char * /*id*/, unsigned long long &/*rev*/) {}
+  template<typename Type>
+  void on_attribute(const char * /*id*/, Type &, const utils::field_attributes &/*attr*/ = utils::null_attributes) {}
+  template<class Pointer>
+  void on_belongs_to(const char * /*id*/, Pointer &obj, const utils::foreign_attributes &attr) {}
+  template<class Pointer>
+  void on_has_one(const char * /*id*/, Pointer &obj, const utils::foreign_attributes &attr) {}
+  template<class ContainerType>
+  void on_has_many(ContainerType &, const char *join_column, const utils::foreign_attributes &attr) {}
+  template<class ContainerType>
+  void on_has_many_to_many(const char * /*id*/, ContainerType &/*c*/, const char *join_column, const char *inverse_join_column, const utils::foreign_attributes &/*attr*/)
+  {
+    join_columns_.join_column = join_column;
+    join_columns_.inverse_join_column = inverse_join_column;
+  }
+  template<class ContainerType>
+  void on_has_many_to_many(const char * /*id*/, ContainerType &/*c*/, const utils::foreign_attributes &/*attr*/) {}
+
+private:
+  join_columns join_columns_;
+};
+
+struct entity_query_data {
+  std::string root_table_name;
+  std::string pk_column_;
+  std::vector<column> columns;
+  std::vector<join_data> joins;
+  std::unique_ptr<basic_condition> where_clause;
+};
+
+enum class query_build_error : std::uint8_t {
+  Ok = 0,
+  UnknownType,
+  MissingPrimaryKey,
+  UnexpectedError
+};
+
+class query_builder_exception : public std::exception
+{
+public:
+  explicit query_builder_exception(query_build_error error) : error_(error) {}
+
+  [[nodiscard]] query_build_error error() const { return error_; }
+
+private:
+  const query_build_error error_;
+};
+
 class entity_query_builder
 {
 public:
-  explicit entity_query_builder(const PrimaryKeyType &pk, const schema &scm)
-  : pk_(pk)
-  , schema_(scm) {}
-
-  // determine pk
-  // collect eager relations for joins
-  template<class EntityType>
-  std::optional<query_context> build(connection &db) {
-    EntityType obj;
-    matador::utils::access::process(*this, obj);
+  explicit entity_query_builder(const schema &scm)
+  : schema_(scm) {}
 
+  template<class EntityType, typename PrimaryKeyType>
+  utils::result<entity_query_data, query_build_error> build(const PrimaryKeyType &pk) {
     const auto info = schema_.info<EntityType>();
     if (!info) {
-      return std::nullopt;
+      return utils::error(query_build_error::UnknownType);
+    }
+    pk_ = pk;
+    table_info_stack_.push(info.value());
+    entity_query_data_ = { info->name };
+    EntityType obj;
+    try {
+      matador::utils::access::process(*this, obj);
+
+      return {utils::ok(std::move(entity_query_data_))};
+    } catch (const query_builder_exception &ex) {
+      return {utils::error(ex.error())};
+    } catch (...) {
+      return {utils::error(query_build_error::UnexpectedError)};
+    }
+  }
+
+  template<class EntityType>
+  utils::result<entity_query_data, query_build_error> build() {
+    const auto info = schema_.info<EntityType>();
+    if (!info) {
+      return utils::error(query_build_error::UnknownType);
+    }
+    pk_ = nullptr;
+    table_info_stack_.push(info.value());
+    entity_query_data_ = { info->name };
+    EntityType obj;
+    try {
+      matador::utils::access::process(*this, obj);
+
+      return {utils::ok(std::move(entity_query_data_))};
+    } catch (const query_builder_exception &ex) {
+      return {utils::error(ex.error())};
+    } catch (...) {
+      return {utils::error(query_build_error::UnexpectedError)};
     }
-    columns_.clear();
-    table_name_ = info.value().name;
-    query q(db, schema_);
-    return q.select(column_generator::generate<EntityType>(schema_)).from({table_name_}).build();
-//    auto from_intermediate = q.select<EntityType>().from({"t"});
-//    return {};
   }
 
   template < class V >
   void on_primary_key(const char *id, V &, typename std::enable_if<std::is_integral<V>::value && !std::is_same<bool, V>::value>::type* = 0)
   {
-    auto b = std::is_integral_v<PrimaryKeyType>;
-    std::cout << "is matching primary key: " << std::boolalpha << b << "\n";
-    columns_.emplace_back(table_name_, id, "");
+    push(id);
+    if (!is_root_entity()) {
+      return;
+    }
+    if (pk_.is_null()) {
+      entity_query_data_.pk_column_ = id;
+    } else if (pk_.is_integer()) {
+      entity_query_data_.where_clause = make_condition(column{table_info_stack_.top().name, id, ""} == *pk_.as<V>());
+      entity_query_data_.pk_column_ = id;
+    }
   }
 
-  void on_primary_key(const char *id, std::string &, size_t)
-  {
-    auto b = std::is_same_v<PrimaryKeyType, std::string>;
-    std::cout << "is matching primary key: " << std::boolalpha << b << "\n";
-    columns_.emplace_back(table_name_, id, "");
-  }
-
-  void on_revision(const char *id, unsigned long long &/*rev*/)
-  {
-    columns_.emplace_back(table_name_, id, "");
-  }
+  void on_primary_key(const char *id, std::string &, size_t);
+  void on_revision(const char *id, unsigned long long &/*rev*/);
 
   template<typename Type>
   void on_attribute(const char *id, Type &, const utils::field_attributes &/*attr*/ = utils::null_attributes)
   {
-    columns_.emplace_back(table_name_, id, "");
+    push(id);
   }
 
   template<class Pointer>
-  void on_belongs_to(const char *id, Pointer &, const utils::foreign_attributes &attr)
+  void on_belongs_to(const char *id, Pointer &obj, const utils::foreign_attributes &attr)
   {
-    if (attr.fetch() == utils::fetch_type::EAGER) {
-      column col{id};
-
-      // collect join information (determine primary key of foreign entity)
-    }
-    columns_.emplace_back(table_name_, id, "");
+    on_foreign_object(id, obj, attr);
   }
 
   template<class Pointer>
-  void on_has_one(const char *id, Pointer &, const utils::foreign_attributes &attr)
+  void on_has_one(const char *id, Pointer &obj, const utils::foreign_attributes &attr)
+  {
+    on_foreign_object(id, obj, attr);
+  }
+
+  template<class ContainerType>
+  void on_has_many(ContainerType &, const char *join_column, const utils::foreign_attributes &attr)
   {
     if (attr.fetch() == utils::fetch_type::EAGER) {
-      auto info = schema_.info<typename Pointer::value_type>();
-      if (info) {
-        column lcol(table_name_, id, "");
-//        column rcol(info.value())
+      const auto info = schema_.info<typename ContainerType::value_type::value_type>();
+      if (!info) {
+        throw query_builder_exception{query_build_error::UnknownType};
       }
-      // collect join information
-    }
-    columns_.emplace_back(table_name_, id, "");
-  }
+      table_info_stack_.push(info.value());
+      typename ContainerType::value_type::value_type obj;
+      matador::utils::access::process(*this , obj);
+      table_info_stack_.pop();
 
-  template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const char *, const char *, const utils::foreign_attributes &attr)
-  {
-    if (attr.fetch() == utils::fetch_type::EAGER) {
-      // collect join information
+      auto pk = info->prototype.primary_key();
+      if (!pk) {
+        throw query_builder_exception{query_build_error::MissingPrimaryKey};
+      }
+
+      append_join({table_info_stack_.top().name, table_info_stack_.top().prototype.primary_key()->name()}, {info->name, join_column});
     }
   }
 
   template<class ContainerType>
-  void on_has_many(const char *id, ContainerType &c, const utils::foreign_attributes &attr)
+  void on_has_many_to_many(const char *id, ContainerType &c, const char *join_column, const char *inverse_join_column, const utils::foreign_attributes &attr)
   {
-    on_has_many(id, c, "", "", attr);
+    if (attr.fetch() != utils::fetch_type::EAGER) {
+      return;
+    }
+    const auto info = schema_.info<typename ContainerType::value_type::value_type>();
+    if (!info) {
+      throw query_builder_exception{query_build_error::UnknownType};
+    }
+    table_info_stack_.push(info.value());
+    typename ContainerType::value_type::value_type obj;
+    matador::utils::access::process(*this , obj);
+    table_info_stack_.pop();
+
+    auto pk = info->prototype.primary_key();
+    if (!pk) {
+      throw query_builder_exception{query_build_error::MissingPrimaryKey};
+    }
+
+    append_join({table_info_stack_.top().name, table_info_stack_.top().prototype.primary_key()->name()}, {id, join_column});
+    append_join({id, inverse_join_column}, {info->name, pk->name()});
+  }
+
+  template<class ContainerType>
+  void on_has_many_to_many(const char *id, ContainerType &c, const utils::foreign_attributes &attr)
+  {
+    if (attr.fetch() != utils::fetch_type::EAGER) {
+      return;
+    }
+    const auto info = schema_.info<typename ContainerType::value_type::value_type>();
+    if (!info) {
+      throw query_builder_exception{query_build_error::UnknownType};
+    }
+    table_info_stack_.push(info.value());
+    typename ContainerType::value_type::value_type obj;
+    matador::utils::access::process(*this , obj);
+    table_info_stack_.pop();
+
+    auto pk = info->prototype.primary_key();
+    if (!pk) {
+      throw query_builder_exception{query_build_error::MissingPrimaryKey};
+    }
+
+    const auto join_columns = join_column_collector_.collect<typename ContainerType::value_type::value_type>();
+
+    append_join({table_info_stack_.top().name, table_info_stack_.top().prototype.primary_key()->name()}, {id, join_columns.inverse_join_column});
+    append_join({id, join_columns.join_column}, {info->name, pk->name()});
   }
 
 private:
-  PrimaryKeyType pk_;
-  std::vector<column> columns_;
+  template<class Pointer>
+  void on_foreign_object(const char *id, Pointer &, const utils::foreign_attributes &attr);
+  void push(const std::string &column_name);
+  [[nodiscard]] bool is_root_entity() const;
+  void append_join(const column &left, const column &right);
+
+private:
+  value pk_;
+  std::stack<table_info> table_info_stack_;
   const schema &schema_;
-  std::string table_name_;
+  entity_query_data entity_query_data_;
+  int column_index{0};
+  join_column_collector join_column_collector_;
 };
 
+template<class Pointer>
+void entity_query_builder::on_foreign_object(const char *id, Pointer &, const utils::foreign_attributes &attr)
+{
+  if (attr.fetch() == utils::fetch_type::EAGER) {
+    const auto info = schema_.info<typename Pointer::value_type>();
+    if (!info) {
+      throw query_builder_exception{query_build_error::UnknownType};
+    }
+    table_info_stack_.push(info.value());
+    typename Pointer::value_type obj;
+    matador::utils::access::process(*this, obj);
+    table_info_stack_.pop();
+
+    auto pk = info->prototype.primary_key();
+    if (!pk) {
+      throw query_builder_exception{query_build_error::MissingPrimaryKey};
+    }
+    append_join({table_info_stack_.top().name, id}, {info->name, pk->name()});
+  } else {
+    push(id);
+  }
+}
+
 }
 #endif //QUERY_ENTITY_QUERY_BUILDER_HPP

include/matador/sql/field.hpp

@@ -1,11 +1,7 @@
 #ifndef QUERY_FIELD_HPP
 #define QUERY_FIELD_HPP
 
-#include "matador/sql/any_type.hpp"
-#include "matador/sql/any_type_to_visitor.hpp"
-#include "matador/sql/data_type_traits.hpp"
-
-#include "matador/utils/types.hpp"
+#include "matador/sql/value.hpp"
 
 #include <optional>
 #include <string>
@@ -19,10 +15,8 @@ public:
   template<typename Type>
   field(std::string name, Type value, size_t size = 0, int index = -1)
   : name_(std::move(name))
-  , size_(size)
   , index_(index)
-  , value_(value)
-  , type_(data_type_traits<Type>::builtin_type(size)) {}
+  , value_(value, size) {}
   field(std::string name, data_type_t data_type, size_t size = 0, int index = -1);
   field(const field &x) = default;
   field& operator=(const field &x) = default;
@@ -32,7 +26,7 @@ public:
   template<typename Type>
   field& operator=(Type value) {
     value_ = std::move(value);
-    type_ = data_type_traits<Type>::builtin_type(-1);
+
     return *this;
   }
 
@@ -43,9 +37,7 @@ public:
   template<class Type>
   std::optional<Type> as() const
   {
-    any_type_to_visitor<Type> visitor;
-    std::visit(visitor, const_cast<any_type&>(value_));
-    return visitor.result;
+    return value_.as<Type>();
   }
 
   [[nodiscard]] std::string str() const;
@@ -65,7 +57,7 @@ private:
   template<class Operator>
   void process(Operator &op)
   {
-    op.on_attribute(name_.c_str(), value_, type_, size_);
+    op.on_attribute(name_.c_str(), value_, value_.size());
   }
 
 private:
@@ -73,9 +65,8 @@ private:
 
   std::string name_;
   int index_{-1};
-  any_type value_;
-  size_t size_{};
-  data_type_t type_;
+
+  value value_;
 };
 
 }

include/matador/sql/has_many_to_many_relation.hpp

@@ -0,0 +1,38 @@
+#ifndef QUERY_HAS_MANY_TO_MANY_RELATION_HPP
+#define QUERY_HAS_MANY_TO_MANY_RELATION_HPP
+
+#include "matador/sql/entity.hpp"
+
+#include "matador/utils/access.hpp"
+#include "matador/utils/foreign_attributes.hpp"
+
+namespace matador::sql {
+
+template < class LocalType, class ForeignType >
+class has_many_to_many_relation
+{
+public:
+  has_many_to_many_relation() = default;
+  has_many_to_many_relation(std::string local_name, std::string remote_name)
+  : local_name_(std::move(local_name))
+  , remote_name_(std::move(remote_name)) {}
+
+  template<class Operator>
+  void process(Operator &op) {
+    namespace field = matador::utils::access;
+    field::belongs_to(op, local_name_.c_str(), local_, utils::default_foreign_attributes);
+    field::belongs_to(op, remote_name_.c_str(), remote_, utils::default_foreign_attributes);
+  }
+
+  entity<LocalType> local() const { return local_; }
+  entity<LocalType> remote() const { return remote_; }
+
+private:
+  std::string local_name_;
+  std::string remote_name_;
+  sql::entity<LocalType> local_;
+  sql::entity<ForeignType> remote_;
+};
+
+}
+#endif //QUERY_HAS_MANY_TO_MANY_RELATION_HPP

include/matador/sql/query_intermediates.hpp

@@ -34,13 +34,13 @@ protected:
 class query_intermediate : public basic_query_intermediate
 {
 public:
-  query_intermediate(connection &db, const sql::schema &schema, query_data &data);
+  query_intermediate(connection &db, const sql::schema &schema, const std::shared_ptr<query_data> &data);
 
 protected:
-  query_data &data_;
+  std::shared_ptr<query_data> data_;
 };
 
-class query_execute_finish : public query_intermediate
+class query_execute : public query_intermediate
 {
 public:
   using query_intermediate::query_intermediate;
@@ -50,7 +50,7 @@ public:
   [[nodiscard]] query_context build() const;
 };
 
-class query_select_finish : public query_intermediate
+class query_select : public query_intermediate
 {
 protected:
   using query_intermediate::query_intermediate;
@@ -64,15 +64,15 @@ public:
   query_result<record> fetch_all();
 
   template < class Type >
-  std::optional<Type> fetch_one()
+  std::unique_ptr<Type> fetch_one()
   {
     auto result = query_result<Type>(fetch());
     auto first = result.begin();
     if (first == result.end()) {
-      return std::nullopt;
+      return nullptr;
     }
 
-    return *first.get();
+    return std::unique_ptr<Type>{first.release()};
   }
   std::optional<record> fetch_one();
 
@@ -96,36 +96,36 @@ private:
 
 class query_offset_intermediate;
 
-class query_limit_intermediate : public query_select_finish
+class query_limit_intermediate : public query_select
 {
 public:
-  using query_select_finish::query_select_finish;
+  using query_select::query_select;
 
   query_offset_intermediate offset(size_t offset);
 };
 
-class query_offset_intermediate : public query_select_finish
+class query_offset_intermediate : public query_select
 {
 public:
-  using query_select_finish::query_select_finish;
+  using query_select::query_select;
 
   query_limit_intermediate limit(size_t limit);
 };
 
-class query_order_direction_intermediate : public query_select_finish
+class query_order_direction_intermediate : public query_select
 {
 public:
-  using query_select_finish::query_select_finish;
+  using query_select::query_select;
 
   query_limit_intermediate limit(size_t limit);
 };
 
 class query_order_by_intermediate;
 
-class query_group_by_intermediate : public query_select_finish
+class query_group_by_intermediate : public query_select
 {
 public:
-  using query_select_finish::query_select_finish;
+  using query_select::query_select;
 
   query_order_by_intermediate order_by(const column &col);
 };
@@ -139,10 +139,10 @@ public:
   query_order_direction_intermediate desc();
 };
 
-class query_where_intermediate : public query_select_finish
+class query_where_intermediate : public query_select
 {
 public:
-  using query_select_finish::query_select_finish;
+  using query_select::query_select;
 
   query_group_by_intermediate group_by(const column &col);
   query_order_by_intermediate order_by(const column &col);
@@ -150,17 +150,30 @@ public:
 
 class query_join_intermediate;
 
-class query_on_intermediate : public query_select_finish
+struct join_data
+{
+  table join_table;
+  std::unique_ptr<basic_condition> condition;
+};
+
+class query_from_intermediate : public query_select
 {
 public:
-  using query_select_finish::query_select_finish;
+  using query_select::query_select;
 
   query_join_intermediate join_left(const table &t);
+  query_from_intermediate join_left(join_data &data);
+  query_from_intermediate join_left(std::vector<join_data> &data_vector);
+
   template<class Condition>
   query_where_intermediate where(const Condition &cond)
   {
     return where_clause(std::make_unique<Condition>(std::move(cond)));
   }
+  query_where_intermediate where(std::unique_ptr<basic_condition> &&cond)
+  {
+    return where_clause(std::move(cond));
+  }
   query_group_by_intermediate group_by(const column &col);
   query_order_by_intermediate order_by(const column &col);
 
@@ -168,6 +181,8 @@ private:
   query_where_intermediate where_clause(std::unique_ptr<basic_condition> &&cond);
 };
 
+using query_on_intermediate = query_from_intermediate;
+
 class query_join_intermediate : public query_intermediate
 {
 public:
@@ -178,36 +193,22 @@ public:
   {
     return on_clause(std::make_unique<Condition>(std::move(cond)));
   }
+  query_on_intermediate on(std::unique_ptr<basic_condition> &&cond)
+  {
+    return on_clause(std::move(cond));
+  }
 
 private:
   query_on_intermediate on_clause(std::unique_ptr<basic_condition> &&cond);
 };
 
-class query_from_intermediate : public query_select_finish
-{
-public:
-  using query_select_finish::query_select_finish;
-
-  query_join_intermediate join_left(const table &t);
-  template<class Condition>
-  query_where_intermediate where(const Condition &cond)
-  {
-    return where_clause(std::make_unique<Condition>(std::move(cond)));
-  }
-  query_group_by_intermediate group_by(const column &col);
-  query_order_by_intermediate order_by(const column &col);
-
-private:
-  query_where_intermediate where_clause(std::unique_ptr<basic_condition> &&cond);
-};
-
 class query_start_intermediate : public basic_query_intermediate
 {
 public:
   explicit query_start_intermediate(connection &db, const sql::schema &schema);
 
 protected:
-  query_data data_;
+  std::shared_ptr<query_data> data_ { std::make_shared<query_data>() };
 };
 
 class query_select_intermediate : public query_start_intermediate
@@ -232,16 +233,16 @@ class query_into_intermediate : public query_intermediate
 public:
   using query_intermediate::query_intermediate;
 
-  query_execute_finish values(std::initializer_list<any_type> values);
-  query_execute_finish values(std::vector<any_type> &&values);
+  query_execute values(std::initializer_list<any_type> values);
+  query_execute values(std::vector<any_type> &&values);
   template<class Type>
-  query_execute_finish values()
+  query_execute values()
   {
     Type obj;
     return values(std::move(as_placeholder(obj)));
   }
   template<class Type>
-  query_execute_finish values(const Type &obj)
+  query_execute values(const Type &obj)
   {
     return values(std::move(value_extractor::extract(obj)));
   }
@@ -252,10 +253,10 @@ class query_create_intermediate : public query_start_intermediate
 public:
   explicit query_create_intermediate(connection &db, const sql::schema &schema);
 
-  query_execute_finish table(const sql::table &table, std::initializer_list<column_definition> columns);
-  query_execute_finish table(const sql::table &table, const std::vector<column_definition> &columns);
+  query_execute table(const sql::table &table, std::initializer_list<column_definition> columns);
+  query_execute table(const sql::table &table, const std::vector<column_definition> &columns);
   template<class Type>
-  query_execute_finish table(const sql::table &table)
+  query_execute table(const sql::table &table)
   {
     return this->table(table, column_definition_generator::generate<Type>(schema_));
   }
@@ -266,7 +267,7 @@ class query_drop_intermediate : query_start_intermediate
 public:
   explicit query_drop_intermediate(connection &db, const sql::schema &schema);
 
-  query_execute_finish table(const sql::table &table);
+  query_execute table(const sql::table &table);
 };
 
 class query_insert_intermediate : public query_start_intermediate
@@ -279,18 +280,18 @@ public:
   query_into_intermediate into(const sql::table &table);
 };
 
-class query_execute_where_intermediate : public query_execute_finish
+class query_execute_where_intermediate : public query_execute
 {
 public:
-  using query_execute_finish::query_execute_finish;
+  using query_execute::query_execute;
 
   query_order_by_intermediate order_by(const column &col);
 };
 
-class query_set_intermediate : public query_execute_finish
+class query_set_intermediate : public query_execute
 {
 public:
-  using query_execute_finish::query_execute_finish;
+  using query_execute::query_execute;
 
   template<class Condition>
   query_execute_where_intermediate where(const Condition &cond)
@@ -316,10 +317,10 @@ public:
   }
 };
 
-class query_delete_from_intermediate : public query_execute_finish
+class query_delete_from_intermediate : public query_execute
 {
 public:
-  using query_execute_finish::query_execute_finish;
+  using query_execute::query_execute;
 
   template<class Condition>
   query_execute_where_intermediate where(const Condition &cond)

include/matador/sql/query_result_impl.hpp

@@ -15,6 +15,8 @@
 
 namespace matador::sql {
 
+class value;
+
 namespace detail {
 class pk_reader
 {
@@ -41,9 +43,11 @@ public:
   void on_has_one(const char * /*id*/, Pointer &/*x*/, const utils::foreign_attributes &/*attr*/) {}
 
   template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const char *, const char *, const utils::foreign_attributes &/*attr*/) {}
+  void on_has_many(ContainerType &, const char *, const utils::foreign_attributes &/*attr*/) {}
   template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const utils::foreign_attributes &/*attr*/) {}
+  void on_has_many_to_many(const char *id, ContainerType &c, const char *join_column, const char *inverse_join_column, const utils::foreign_attributes &/*attr*/) {}
+  template<class ContainerType>
+  void on_has_many_to_many(const char *id, ContainerType &c, const utils::foreign_attributes &/*attr*/) {}
 
 private:
   size_t column_index_{};
@@ -72,7 +76,7 @@ public:
   }
   void on_attribute(const char *id, char *value, const utils::field_attributes &attr = utils::null_attributes);
   void on_attribute(const char *id, std::string &value, const utils::field_attributes &attr = utils::null_attributes);
-  void on_attribute(const char *id, any_type &value, data_type_t type, const utils::field_attributes &attr = utils::null_attributes);
+  void on_attribute(const char *id, value &val, const utils::field_attributes &attr = utils::null_attributes);
 
   template < class Pointer >
   void on_belongs_to(const char * /*id*/, Pointer &x, const utils::foreign_attributes &attr)

include/matador/sql/query_result_reader.hpp

@@ -6,6 +6,8 @@
 
 namespace matador::sql {
 
+class value;
+
 class query_result_reader
 {
 public:
@@ -34,7 +36,7 @@ public:
   virtual void read_value(const char *id, size_t index, std::string &value);
   virtual void read_value(const char *id, size_t index, std::string &value, size_t s);
   virtual void read_value(const char *id, size_t index, utils::blob &value);
-  virtual void read_value(const char *id, size_t index, any_type &value, data_type_t type, size_t size);
+  virtual void read_value(const char *id, size_t index, value &val, size_t size);
 };
 
 }

include/matador/sql/record.hpp

@@ -15,8 +15,9 @@ struct column;
 class record
 {
 private:
-  using field_by_index = std::vector<field>;
-  using field_index_pair = std::pair<std::reference_wrapper<field>, field_by_index::difference_type>;
+  using field_ref = std::reference_wrapper<field>;
+  using field_by_index = std::vector<field_ref>;
+  using field_index_pair = std::pair<field, field_by_index::difference_type>;
   using field_by_name_map = std::unordered_map<std::string, field_index_pair>;
 
 public:
@@ -36,13 +37,13 @@ public:
   void process(Operator &op)
   {
     for(auto &f : fields_) {
-      f.process(op);
+      f.get().process(op);
     }
   }
 
-  void append(field col);
+  void append(const field &col);
 
-  [[nodiscard]] const std::vector<field>& columns() const;
+  [[nodiscard]] const std::vector<field_ref>& columns() const;
 
   [[nodiscard]] const field& at(const column &col) const;
   [[nodiscard]] const field& at(size_t index) const;

include/matador/sql/schema.hpp

@@ -23,6 +23,7 @@ class schema
 {
 public:
   using repository = std::unordered_map<std::type_index, table_info>;
+  using repository_by_name = std::unordered_map<std::string, std::reference_wrapper<table_info>>;
   using iterator = repository::iterator;
   using const_iterator = repository::const_iterator;
 
@@ -35,6 +36,8 @@ public:
 
   [[nodiscard]] std::string name() const;
 
+  void create(connection &c);
+
   template<typename Type>
   const table_info& attach(const std::string &table_name)
   {
@@ -50,6 +53,7 @@ public:
   }
 
   [[nodiscard]] std::optional<table_info> info(std::type_index ti) const;
+  [[nodiscard]] std::optional<table_info> info(const std::string &name) const;
 
   template<typename Type>
   [[nodiscard]] std::pair<std::string, std::string> reference() const
@@ -77,6 +81,7 @@ public:
 private:
   std::string name_;
   repository repository_;
+  repository_by_name repository_by_name_;
 };
 
 }

include/matador/sql/session.hpp

@@ -4,6 +4,7 @@
 #include "matador/sql/connection.hpp"
 #include "matador/sql/connection_pool.hpp"
 #include "matador/sql/entity.hpp"
+#include "matador/sql/entity_query_builder.hpp"
 #include "matador/sql/statement.hpp"
 #include "matador/sql/schema.hpp"
 
@@ -13,6 +14,14 @@ namespace matador::sql {
 
 class dialect;
 
+enum class session_error {
+  Ok = 0,
+  NoConnectionAvailable,
+  UnknownType,
+  FailedToBuildQuery,
+  FailedToFindObject
+};
+
 class session
 {
 public:
@@ -32,30 +41,68 @@ public:
   }
 
   template<typename Type, typename PrimaryKeyType>
-  entity<Type> find(const PrimaryKeyType &pk) {
+  utils::result<entity<Type>, session_error> find(const PrimaryKeyType &pk) {
     auto c = pool_.acquire();
     if (!c.valid()) {
-      throw std::logic_error("no database connection available");
+      return utils::error(session_error::NoConnectionAvailable);
     }
-    // collect all columns
-    // - evaluate fetch::Eager flag for relations
     auto info = schema_->info<Type>();
     if (!info) {
-      return {};
+      return utils::error(session_error::UnknownType);
     }
 
-    c->query(*schema_).select({}).from(info->name);
-    // build pk where condition
-    // - check if type has pk
-    // - check type
-//    pk_condition_builder<Type> builder;
-//    auto cond = builder.build(pk);
+    entity_query_builder eqb(*schema_);
+    auto data = eqb.build<Type>(pk);
+    if (!data.is_ok()) {
+      return utils::error(session_error::FailedToBuildQuery);
+    }
 
-    // create query with relations as requested
-    // 
-//    c->query(*schema_).select<Type>().from("xyz").where().fetch_all();
+    auto obj = build_select_query(c, data.release()).template fetch_one<Type>();
 
-    return {};
+    if (!obj) {
+      return utils::error(session_error::FailedToFindObject);
+    }
+    return utils::ok(entity<Type>{ obj.release() });
+  }
+
+  template<typename Type>
+  utils::result<query_result<Type>, session_error> find() {
+    auto c = pool_.acquire();
+    if (!c.valid()) {
+      return utils::error(session_error::NoConnectionAvailable);
+    }
+    auto info = schema_->info<Type>();
+    if (!info) {
+      return utils::error(session_error::UnknownType);
+    }
+
+    entity_query_builder eqb(*schema_);
+    auto data = eqb.build<Type>();
+    if (!data.is_ok()) {
+      return utils::error(session_error::FailedToBuildQuery);
+    }
+
+    return utils::ok(build_select_query(c, data.release()).template fetch_all<Type>());
+  }
+
+  template<typename Type>
+  utils::result<query_from_intermediate, session_error> select() {
+    auto c = pool_.acquire();
+    if (!c.valid()) {
+      return utils::error(session_error::NoConnectionAvailable);
+    }
+    auto info = schema_->info<Type>();
+    if (!info) {
+      return utils::error(session_error::UnknownType);
+    }
+
+    entity_query_builder eqb(*schema_);
+    auto data = eqb.build<Type>();
+    if (!data.is_ok()) {
+      return utils::error(session_error::FailedToBuildQuery);
+    }
+
+    return utils::ok(build_select_query(c, data.release()).template fetch_all<Type>());
   }
 
   template<typename Type>
@@ -70,15 +117,15 @@ public:
   std::vector<sql::column_definition> describe_table(const std::string &table_name) const;
   bool table_exists(const std::string &table_name) const;
 
-  [[nodiscard]] const schema& tables() const;
-
   const class dialect& dialect() const;
 
 private:
-  friend class query_select_finish;
+  friend class query_select;
 
   [[nodiscard]] std::unique_ptr<query_result_impl> fetch(const std::string &sql) const;
 
+  query_select build_select_query(connection_ptr<connection> &conn, entity_query_data &&data) const;
+
 private:
   connection_pool<connection> &pool_;
   const class dialect &dialect_;
@@ -103,7 +150,7 @@ entity<Type> session::insert(Type *obj)
   }
   c->query(*schema_)
     .insert()
-    .into(info->name, column_generator::generate<Type>(*schema_))
+    .into(info->name, column_generator::generate<Type>(*schema_, true))
     .values(*obj)
     .execute();
 

include/matador/sql/table.hpp

@@ -14,13 +14,22 @@ struct table
   table(const char *name, std::string as = "") // NOLINT(*-explicit-constructor)
   : name(name), alias(std::move(as)) {}
   table(std::string name, std::string as = "") // NOLINT(*-explicit-constructor)
-  : name(std::move(name)), alias(std::move(as)) {}
+  : name(std::move(name))
+  , alias(std::move(as)) {}
+  table(std::string name, std::string as, const std::vector<column> &columns)
+  : name(std::move(name))
+  , alias(std::move(as))
+  , columns(columns) {}
 
   table& as(const std::string &a) {
     alias = a;
     return *this;
   }
 
+  [[nodiscard]] table as(const std::string &a) const {
+    return { name, a, columns };
+  }
+
   std::string name;
   std::string alias;
 

include/matador/sql/value.hpp

@@ -0,0 +1,81 @@
+#ifndef QUERY_VALUE_HPP
+#define QUERY_VALUE_HPP
+
+#include "matador/sql/any_type.hpp"
+#include "matador/sql/any_type_to_visitor.hpp"
+#include "matador/sql/data_type_traits.hpp"
+
+#include "matador/utils/types.hpp"
+
+#include <optional>
+
+namespace matador::sql {
+
+namespace detail {
+template<typename Type>
+size_t determine_size(const Type &/*val*/)
+{
+  return 0;
+}
+size_t determine_size(const std::string &val);
+size_t determine_size(const char *val);
+size_t determine_size(const utils::blob &val);
+
+}
+class value
+{
+public:
+  value() = default;
+  template<typename Type>
+  explicit value(Type value, size_t size = 0)
+  : size_(size)
+  , value_(value)
+  , type_(data_type_traits<Type>::builtin_type(size)) {}
+  explicit value(data_type_t data_type, size_t size = 0);
+  value(const value &x) = default;
+  value& operator=(const value &x) = default;
+  template<typename Type>
+  value& operator=(Type val)
+  {
+    value_ = val;
+    size_ = detail::determine_size(val);
+    type_ = data_type_traits<Type>::builtin_type(size_);
+    return *this;
+  }
+  value(value &&x) noexcept;
+  value& operator=(value &&x) noexcept;
+
+  template<class Type>
+  std::optional<Type> as() const
+  {
+    if (std::holds_alternative<Type>(value_)) {
+      return std::get<Type>(value_);
+    } else {
+      any_type_to_visitor<Type> visitor;
+      std::visit(visitor, const_cast<any_type &>(value_));
+      return visitor.result;
+    }
+  }
+  [[nodiscard]] std::string str() const;
+
+  [[nodiscard]] size_t size() const;
+  [[nodiscard]] data_type_t type() const;
+
+  [[nodiscard]] bool is_integer() const;
+  [[nodiscard]] bool is_floating_point() const;
+  [[nodiscard]] bool is_bool() const;
+  [[nodiscard]] bool is_string() const;
+  [[nodiscard]] bool is_varchar() const;
+  [[nodiscard]] bool is_blob() const;
+  [[nodiscard]] bool is_null() const;
+  [[nodiscard]] bool is_unknown() const;
+
+private:
+  any_type value_;
+  size_t size_{};
+  data_type_t type_{data_type_t::type_unknown};
+
+};
+
+}
+#endif //QUERY_VALUE_HPP

include/matador/sql/value_extractor.hpp

@@ -53,9 +53,11 @@ public:
     values_.emplace_back(fk_value_extractor_.extract(*x));
   }
   template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const char *, const char *, const utils::foreign_attributes &/*attr*/) {}
+  void on_has_many(ContainerType &, const char *, const utils::foreign_attributes &/*attr*/) {}
   template<class ContainerType>
-  void on_has_many(const char *, ContainerType &, const utils::foreign_attributes &/*attr*/) {}
+  void on_has_many_to_many(const char *id, ContainerType &c, const char *join_column, const char *inverse_join_column, const utils::foreign_attributes &/*attr*/) {}
+  template<class ContainerType>
+  void on_has_many_to_many(const char *id, ContainerType &c, const utils::foreign_attributes &/*attr*/) {}
 
 private:
   template<typename Type>

include/matador/utils/access.hpp

@@ -64,24 +64,19 @@ void belongs_to(Operator &op, const char *id, Type &value, const foreign_attribu
   op.on_belongs_to(id, value, attr);
 }
 
-//template<class Operator, class Type, template<class ...> class ContainerType>
-//void has_many(Operator &op, const char *id, container<Type, ContainerType> &container, const foreign_attributes &attr) {
-//  op.on_has_many(id, container, attr);
-//}
-
 template<class Operator, class Type, template<class ...> class ContainerType>
-void has_many(Operator &op, const char *id, ContainerType<Type> &container, const foreign_attributes &attr) {
-  op.on_has_many(id, container, attr);
+void has_many(Operator &op, ContainerType<Type> &container, const char *join_column, const foreign_attributes &attr) {
+  op.on_has_many(container, join_column, attr);
 }
 
-//template<class Operator, class Type, template<class ...> class ContainerType>
-//void has_many(Operator &op, const char *id, container<Type, ContainerType> &container, const char *left_column, const char *right_column, const foreign_attributes &attr) {
-//  op.on_has_many(id, container, left_column, right_column, attr);
-//}
+template<class Operator, class Type, template<class ...> class ContainerType>
+void has_many_to_many(Operator &op, const char *id, ContainerType<Type> &container, const char *join_column, const char *inverse_join_column, const foreign_attributes &attr) {
+  op.on_has_many_to_many(id, container, join_column, inverse_join_column, attr);
+}
 
 template<class Operator, class Type, template<class ...> class ContainerType>
-void has_many(Operator &op, const char *id, ContainerType<Type> &container, const char *left_column, const char *right_column, const foreign_attributes &attr) {
-  op.on_has_many(id, container, left_column, right_column, attr);
+void has_many_to_many(Operator &op, const char *id, ContainerType<Type> &container, const foreign_attributes &attr) {
+  op.on_has_many_to_many(id, container, attr);
 }
 
 }

include/matador/utils/result.hpp

@@ -0,0 +1,113 @@
+#ifndef QUERY_RESULT_HPP
+#define QUERY_RESULT_HPP
+
+#include <variant>
+
+namespace matador::utils {
+
+template < typename ValueType, typename ErrorType >
+class [[nodiscard]] result;
+
+template <typename ValueType>
+struct is_result : std::false_type {};
+
+template < typename ValueType, typename ErrorType >
+struct is_result<result<ValueType, ErrorType>> : std::true_type {};
+
+template < typename ValueType >
+class ok
+{
+public:
+  using value_type = ValueType;
+  explicit constexpr ok(const ValueType &value) : value_(value) {}
+  explicit constexpr ok(ValueType &&value) : value_(std::move(value)) {}
+
+  constexpr ValueType&& release() { return std::move(value_); }
+  const ValueType& value() const { return value_; }
+  ValueType& value() { return value_; }
+
+private:
+  ValueType value_;
+};
+
+template < typename ErrorType >
+class error
+{
+public:
+  using value_type = ErrorType;
+
+  explicit constexpr error(const ErrorType &error) : error_(error) {}
+  explicit constexpr error(ErrorType &&error) : error_(std::move(error)) {}
+
+  constexpr ErrorType&& release() { return std::move(error_); }
+  const ErrorType& value() const { return error_; }
+  ErrorType value() { return error_; }
+
+private:
+  ErrorType error_;
+};
+
+template < typename ValueType, typename ErrorType >
+class result
+{
+public:
+  using value_type = ok<ValueType>;
+  using error_type = error<ErrorType>;
+
+  result() : result_(ValueType{}) {}
+  result(value_type value) : result_(std::move(value)) {} // NOLINT(*-explicit-constructor)
+  result(error_type error) : result_(std::move(error)) {} // NOLINT(*-explicit-constructor)
+  result(const result<ValueType, ErrorType> &x) = default;
+  result& operator=(const result<ValueType, ErrorType> &x) = default;
+  result(result<ValueType, ErrorType> &&x) = default;
+  result& operator=(result<ValueType, ErrorType> &&x) = default;
+
+  operator bool() const { return is_ok(); } // NOLINT(*-explicit-constructor)
+
+  [[nodiscard]] bool is_ok() const { return std::holds_alternative<value_type>(result_); }
+  [[nodiscard]] bool is_error() const { return std::holds_alternative<error_type>(result_); }
+
+  ValueType&& release() { return std::get<value_type>(result_).release(); }
+  ErrorType&& release_error() { return std::get<error_type>(result_).release(); }
+
+  const ValueType& value() const { return std::get<value_type>(result_).value(); }
+  const ErrorType& err() const { return std::get<error_type>(result_).value(); }
+  ErrorType err() { return std::get<error_type>(result_).value(); }
+
+  constexpr const ValueType* operator->() const { return &value(); }
+  constexpr ValueType* operator->() { return &std::get<value_type>(result_).value(); }
+
+  template<typename Func, typename SecondValueType = typename std::invoke_result_t<Func, ValueType >>
+  result<SecondValueType, ErrorType> transform(Func &&f) {
+    if (is_ok()) {
+      return result<SecondValueType, ErrorType>(ok(f(release())));
+    }
+
+    return result<SecondValueType, ErrorType>(error(release_error()));
+  }
+
+  template<typename Func, typename SecondValueType = typename std::invoke_result_t<Func, ValueType >::value_type::value_type>
+  result<SecondValueType, ErrorType> and_then(Func &&f) {
+    if (is_ok()) {
+      return f(release());
+    }
+
+    return result<SecondValueType, ErrorType>(error(release_error()));
+  }
+
+  template<typename Func, typename SecondErrorType = typename std::invoke_result_t<Func, ErrorType >::error_type::value_type>
+  result<ValueType, SecondErrorType> or_else(Func &&f) {
+    if (is_error()) {
+      return f(err());
+    }
+
+    return result<ValueType, SecondErrorType>(ok(release()));
+  }
+
+private:
+  std::variant<value_type, error_type> result_;
+};
+
+}
+
+#endif //QUERY_RESULT_HPP

src/CMakeLists.txt

@@ -37,6 +37,8 @@ set(SQL_SOURCES
   sql/any_type_to_string_visitor.cpp
   sql/field.cpp
   sql/table_definition.cpp
+  sql/value.cpp
+  sql/entity_query_builder.cpp
 )
 
 set(SQL_HEADER
@@ -91,28 +93,8 @@ set(SQL_HEADER
   ../include/matador/sql/field.hpp
   ../include/matador/sql/entity_query_builder.hpp
   ../include/matador/sql/table_definition.hpp
-)
-
-set(QUERY_SOURCES
-  query/query.cpp
-  query/query_compiler.cpp
-  query/query_parts.cpp
-  query/column.cpp
-  query/query_builder.cpp
-  query/query_intermediates.cpp
-)
-
-set(QUERY_HEADER
-  ../include/matador/query/query.hpp
-  ../include/matador/query/query_part_visitor.hpp
-  ../include/matador/query/query_compiler.hpp
-  ../include/matador/query/query_parts.hpp
-  ../include/matador/query/column.hpp
-  ../include/matador/query/query_builder.hpp
-  ../include/matador/query/query_intermediates.hpp
-  ../include/matador/query/sql_commands.hpp
-  ../include/matador/query/query_data.hpp
-  ../include/matador/query/table.hpp
+  ../include/matador/sql/has_many_to_many_relation.hpp
+  ../include/matador/sql/value.hpp
 )
 
 set(UTILS_HEADER
@@ -129,6 +111,7 @@ set(UTILS_HEADER
   ../include/matador/utils/types.hpp
   ../include/matador/utils/foreign_attributes.hpp
   ../include/matador/utils/fetch_type.hpp
+  ../include/matador/utils/result.hpp
 )
 
 set(UTILS_SOURCES

src/query/column.cpp

@@ -1,21 +0,0 @@
-#include "matador/query/column.hpp"
-
-namespace matador::query {
-
-column::column(const char *name)
-: name(name) {}
-
-column::column(std::string name)
-: name(std::move(name)) {}
-
-column::column(std::string table_name, std::string name, std::string as)
-: table(std::move(table_name))
-, name(std::move(name))
-, alias(std::move(as)) {}
-
-column& column::as(std::string a) {
-  alias = std::move(a);
-  return *this;
-}
-
-}

src/query/query.cpp

@@ -1,6 +0,0 @@
-#include "matador/query/query.hpp"
-
-namespace matador::sql {
-
-
-}

src/query/query_builder.cpp

@@ -1,13 +0,0 @@
-#include "matador/query/query_builder.hpp"
-
-namespace matador::query {
-
-query_builder::query_builder(const std::shared_ptr<sql::schema> &scm)
-: schema_(scm) {}
-
-query_select_intermediate query_builder::select(std::initializer_list<column> columns)
-{
-  return query_select_intermediate{schema_};
-}
-
-}

src/query/query_compiler.cpp

@@ -1,33 +0,0 @@
-#include "matador/query/query_compiler.hpp"
-#include "matador/query/query_data.hpp"
-#include "matador/query/column.hpp"
-
-#include "matador/sql/dialect.hpp"
-
-namespace matador::query {
-
-query_compiler::query_compiler(sql::dialect &d)
-  : dialect_(d)
-{}
-
-std::string query_compiler::compile(const query_data *data)
-{
-  sql_.clear();
-  for (const auto &part : data->parts) {
-    part->accept(*this);
-  }
-
-  return sql_;
-}
-
-void query_compiler::visit(query_select_part &select_part)
-{
-  auto ci = "name"_col.as("c01");
-}
-
-void query_compiler::visit(query_from_part &from_part)
-{
-
-}
-
-}

src/query/query_intermediates.cpp

@@ -1,8 +0,0 @@
-#include "matador/query/query_intermediates.hpp"
-
-namespace matador::query {
-
-query_intermediate::query_intermediate(const std::shared_ptr<sql::schema> &scm)
-: schema_(scm) {}
-
-}

src/query/query_parts.cpp

@@ -1,30 +0,0 @@
-#include "matador/query/query_parts.hpp"
-
-namespace matador::query {
-
-query_part::query_part(sql::dialect::token_t token)
-  : token_(token) {}
-
-query_select_part::query_select_part(std::vector<column> columns)
-  : query_part(sql::dialect::token_t::SELECT)
-  , columns_(std::move(columns)) {}
-
-void query_select_part::accept(query_part_visitor &visitor)
-{
-  visitor.visit(*this);
-}
-
-query_from_part::query_from_part(std::string table_name)
-  : query_part(sql::dialect::token_t::FROM)
-  , table_(std::move(table_name)) {}
-
-query_from_part::query_from_part(std::string table_name, std::string as)
-  : query_part(sql::dialect::token_t::FROM)
-  , table_(std::move(table_name))
-  , alias_(std::move(as)) {}
-
-void query_from_part::accept(query_part_visitor &visitor)
-{
-  visitor.visit(*this);
-}
-}

src/sql/backend_provider.cpp

@@ -2,6 +2,7 @@
 #include "matador/sql/noop_connection.hpp"
 #include "matador/sql/dialect_builder.hpp"
 
+#include <algorithm>
 #include <stdexcept>
 
 namespace matador::sql {

src/sql/column_definition.cpp

@@ -63,6 +63,46 @@ const std::string &column_definition::ref_column() const
   return ref_column_;
 }
 
+bool column_definition::is_integer() const
+{
+  return type_ >= data_type_t::type_char && type_ <= data_type_t::type_unsigned_long_long;
+}
+
+bool column_definition::is_floating_point() const
+{
+  return type_ == data_type_t::type_float || type_ == data_type_t::type_double;
+}
+
+bool column_definition::is_bool() const
+{
+  return type_ == data_type_t::type_bool;
+}
+
+bool column_definition::is_string() const
+{
+  return type_ == data_type_t::type_text;
+}
+
+bool column_definition::is_varchar() const
+{
+  return type_ == data_type_t::type_varchar;
+}
+
+bool column_definition::is_blob() const
+{
+  return type_ == data_type_t::type_blob;
+}
+
+bool column_definition::is_null() const
+{
+  return type_ == data_type_t::type_null;
+}
+
+bool column_definition::is_unknown() const
+{
+  return type_ == data_type_t::type_unknown;
+}
+
 void column_definition::type(data_type_t type)
 {
   type_ = type;
@@ -70,6 +110,10 @@ void column_definition::type(data_type_t type)
 
 std::string column_definition::str() const
 {
+  if (std::holds_alternative<std::string>(value_)) {
+    return std::get<std::string>(value_);
+  }
+
   any_type_to_visitor<std::string> visitor;
   std::visit(visitor, const_cast<any_type &>(value_));
   return visitor.result;

src/sql/condition.cpp

@@ -1,28 +1,46 @@
 #include "matador/sql/condition.hpp"
 
+#include "matador/sql/query_intermediates.hpp"
+
 namespace matador::sql {
 
 condition<column, placeholder, std::enable_if<true>::type>::condition(const column &fld, basic_condition::operand_t op, const placeholder &val)
 : basic_column_condition(fld, op), value(val)
 {}
 
-std::string condition<column, placeholder, std::enable_if<true>::type>::evaluate(dialect &d, query_context &query) const
+std::string condition<column, placeholder, std::enable_if<true>::type>::evaluate(const dialect &d, query_context &query) const
 {
   query.bind_vars.emplace_back(field_.name);
   return d.prepare_identifier(field_) + " " + operand + " " + d.next_placeholder(query.bind_vars);
 }
 
-condition<column, query_context>::condition(column col, basic_condition::operand_t op, query_context &q)
+condition<column, query_context>::condition(column col, basic_condition::operand_t op, const query_context &q)
 : basic_column_condition(std::move(col), op), query_(q)
 {}
 
-std::string condition<column, query_context>::evaluate(dialect &d, query_context &query) const
+std::string condition<column, query_context>::evaluate(const dialect &d, query_context &query) const
 {
   std::string result(d.prepare_identifier(field_) + " " + operand + " (");
-  result += (")");
+  result += query_.sql + (")");
   return result;
 }
 
+condition<column, query_context> in( const column& col, const query_context &q ) {
+    return {col, basic_condition::operand_t::IN_LIST, q};
+}
+
+condition<column, query_context> in( const column& col, const query_select& q ) {
+    return in(col, q.build());
+}
+
+condition<column, std::string> like( const column& col, const std::string& val ) {
+    return {col, basic_condition::operand_t::LIKE, val};
+}
+
+condition<column, query_context> equals( const column& col, query_context& q ) {
+    return {col, basic_condition::operand_t::EQUAL, q};
+}
+
 condition<column, column> operator==(const column &a, const column &b)
 {
   return {a, basic_condition::operand_t::EQUAL, b};

src/sql/connection.cpp

@@ -91,7 +91,7 @@ size_t connection::execute(const std::string &sql) const
   return connection_->execute(sql);
 }
 
-sql::query connection::query(const sql::schema &schema) const
+sql::query connection::query(const schema &schema) const
 {
   return sql::query(*const_cast<connection*>(this), schema);
 }

src/sql/data_type_traits.cpp

@@ -6,6 +6,21 @@
 
 namespace matador::sql {
 
+void data_type_traits<nullptr_t>::read_value(query_result_reader &reader, const char *id, size_t index, nullptr_t &/*value*/)
+{
+//  reader.read_value(id, index, value);
+}
+
+void data_type_traits<nullptr_t>::bind_value(parameter_binder &binder, size_t index, nullptr_t &/*value*/)
+{
+//  binder.bind(index, value);
+}
+
+void data_type_traits<nullptr_t>::bind_result_value(result_parameter_binder &binder, size_t index, nullptr_t &/*value*/)
+{
+//  binder.bind_result_value(index, value);
+}
+
 void data_type_traits<char>::read_value(query_result_reader &reader, const char *id, size_t index, char &value)
 {
   reader.read_value(id, index, value);

src/sql/dialect.cpp

@@ -51,8 +51,8 @@ std::string dialect::prepare_literal(const std::string &str) const
 
 void dialect::quote_identifier(std::string &str) const
 {
-  str.insert(0, token_at(token_t::START_QUOTE));
-  str += token_at(token_t::END_QUOTE);
+  // str.insert(0, token_at(token_t::START_QUOTE));
+  // str += token_at(token_t::END_QUOTE);
 }
 
 void dialect::escape_quotes_in_identifier(std::string &str) const

src/sql/entity_query_builder.cpp

@@ -0,0 +1,38 @@
+#include "matador/sql/entity_query_builder.hpp"
+
+namespace matador::sql {
+
+void entity_query_builder::on_primary_key(const char *id, std::string &, size_t)
+{
+  push(id);
+  if (!is_root_entity()) {
+    auto b = pk_.is_varchar();
+    std::cout << "is matching primary key: " << std::boolalpha << b << "\n";
+  }
+}
+
+void entity_query_builder::on_revision(const char *id, unsigned long long &/*rev*/)
+{
+  push(id);
+}
+
+void entity_query_builder::push(const std::string &column_name)
+{
+  char str[4];
+  snprintf(str, 4, "c%02d", ++column_index);
+  entity_query_data_.columns.emplace_back(table_info_stack_.top().name, column_name, str);
+}
+
+[[nodiscard]] bool entity_query_builder::is_root_entity() const {
+  return table_info_stack_.size() == 1;
+}
+
+void entity_query_builder::append_join(const column &left, const column &right)
+{
+  entity_query_data_.joins.push_back({
+                                       { right.table },
+                                       make_condition(left == right)
+                                     });
+}
+
+}

src/sql/field.cpp

@@ -7,24 +7,20 @@ namespace matador::sql {
 field::field(std::string name)
   : name_(std::move(name))
   , value_(nullptr)
-  , type_(data_type_t::type_unknown)
 {}
 
 field::field(std::string name, data_type_t data_type, size_t size, int index)
   : name_(std::move(name))
-  , size_(size)
   , index_(index)
-  , type_(data_type) {}
+  , value_(data_type, size) {}
 
 field::field(field &&x) noexcept
   : name_(std::move(x.name_))
   , index_(x.index_)
   , value_(std::move(x.value_))
-  , type_(x.type_)
 {
   x.value_ = nullptr;
   x.index_ = -1;
-  x.type_ = data_type_t::type_unknown;
 }
 
 field &field::operator=(field &&x) noexcept
@@ -32,10 +28,8 @@ field &field::operator=(field &&x) noexcept
   name_ = std::move(x.name_);
   index_ = x.index_;
   value_ = std::move(x.value_);
-  type_ = x.type_;
   x.index_ = -1;
   x.value_ = nullptr;
-  x.type_ = data_type_t::type_unknown;
 
   return *this;
 }
@@ -47,7 +41,7 @@ const std::string &field::name() const
 
 size_t field::size() const
 {
-  return size_;
+  return value_.size();
 }
 
 int field::index() const
@@ -68,42 +62,42 @@ std::string field::str() const
 
 bool field::is_integer() const
 {
-  return type_ >= data_type_t::type_char && type_ <= data_type_t::type_unsigned_long_long;
+  return value_.is_integer();
 }
 
 bool field::is_floating_point() const
 {
-  return type_ == data_type_t::type_float || type_ == data_type_t::type_double;
+  return value_.is_floating_point();
 }
 
 bool field::is_bool() const
 {
-  return type_ == data_type_t::type_bool;
+  return value_.is_bool();
 }
 
 bool field::is_string() const
 {
-  return type_ == data_type_t::type_text;
+  return value_.is_string();
 }
 
 bool field::is_varchar() const
 {
-  return type_ == data_type_t::type_varchar;
+  return value_.is_varchar();
 }
 
 bool field::is_blob() const
 {
-  return type_ == data_type_t::type_blob;
+  return value_.is_blob();
 }
 
 bool field::is_null() const
 {
-  return type_ == data_type_t::type_null;
+  return value_.is_null();
 }
 
 bool field::is_unknown() const
 {
-  return type_ == data_type_t::type_unknown;
+  return value_.is_unknown();
 }
 
 }

src/sql/query_compiler.cpp

@@ -54,7 +54,7 @@ void query_compiler::visit(query_select_part &select_part)
 void query_compiler::visit(query_from_part &from_part)
 {
   query_.table = from_part.table();
-  if (dialect_.default_schema_name().empty()) {
+  if (!dialect_.default_schema_name().empty()) {
     query_.sql += " " + dialect_.token_at(dialect::token_t::FROM) +
                   " " + dialect_.prepare_identifier(from_part.table().name) +
                   (from_part.table().alias.empty() ? "" : " AS " +

src/sql/query_intermediates.cpp

@@ -8,16 +8,16 @@ basic_query_intermediate::basic_query_intermediate(connection &db, const sql::sc
 : connection_(db)
 , schema_(schema) {}
 
-query_result<record> query_select_finish::fetch_all()
+query_result<record> query_select::fetch_all()
 {
   query_compiler compiler(connection_.dialect());
-  return connection_.fetch(compiler.compile(&data_));
+  return connection_.fetch(compiler.compile(data_.get()));
 }
 
-std::optional<record> query_select_finish::fetch_one()
+std::optional<record> query_select::fetch_one()
 {
   query_compiler compiler(connection_.dialect());
-  auto result = connection_.fetch(compiler.compile(&data_));
+  auto result = connection_.fetch(compiler.compile(data_.get()));
   auto first = result.begin();
   if (first == result.end()) {
     return std::nullopt;
@@ -26,145 +26,139 @@ std::optional<record> query_select_finish::fetch_one()
   return *first.get();
 }
 
-query_context query_select_finish::build() const
+query_context query_select::build() const
 {
   query_compiler compiler(connection_.dialect());
-  return compiler.compile(&data_);
+  return compiler.compile(data_.get());
 }
 
-std::unique_ptr<query_result_impl> query_select_finish::fetch()
+std::unique_ptr<query_result_impl> query_select::fetch()
 {
   query_compiler compiler(connection_.dialect());
-  return connection_.fetch(compiler.compile(&data_).sql);
+  return connection_.fetch(compiler.compile(data_.get()).sql);
 }
 
-statement query_select_finish::prepare()
+statement query_select::prepare()
 {
   query_compiler compiler(connection_.dialect());
-  return connection_.prepare(compiler.compile(&data_));
+  return connection_.prepare(compiler.compile(data_.get()));
 }
 
-query_intermediate::query_intermediate(connection &db, const sql::schema &schema, query_data &data)
+query_intermediate::query_intermediate(connection &db, const sql::schema &schema, const std::shared_ptr<query_data> &data)
 : basic_query_intermediate(db, schema), data_(data) {}
 
 query_offset_intermediate query_limit_intermediate::offset(size_t offset)
 {
-  data_.parts.push_back(std::make_unique<query_offset_part>(offset));
+  data_->parts.push_back(std::make_unique<query_offset_part>(offset));
   return {connection_, schema_, data_};
 }
 
 query_limit_intermediate query_offset_intermediate::limit(size_t limit)
 {
-  data_.parts.push_back(std::make_unique<query_limit_part>(limit));
+  data_->parts.push_back(std::make_unique<query_limit_part>(limit));
   return {connection_, schema_, data_};
 }
 
 query_limit_intermediate query_order_direction_intermediate::limit(size_t limit)
 {
-  data_.parts.push_back(std::make_unique<query_limit_part>(limit));
+  data_->parts.push_back(std::make_unique<query_limit_part>(limit));
   return {connection_, schema_, data_};
 }
 
 query_order_by_intermediate query_group_by_intermediate::order_by(const column &col)
 {
-  data_.parts.push_back(std::make_unique<query_order_by_part>(col));
+  data_->parts.push_back(std::make_unique<query_order_by_part>(col));
   return {connection_, schema_, data_};
 }
 
 query_order_direction_intermediate query_order_by_intermediate::asc()
 {
-  data_.parts.push_back(std::make_unique<query_order_by_asc_part>());
+  data_->parts.push_back(std::make_unique<query_order_by_asc_part>());
   return {connection_, schema_, data_};
 }
 
 query_order_direction_intermediate query_order_by_intermediate::desc()
 {
-  data_.parts.push_back(std::make_unique<query_order_by_desc_part>());
+  data_->parts.push_back(std::make_unique<query_order_by_desc_part>());
   return {connection_, schema_, data_};
 }
 
 query_where_intermediate query_from_intermediate::where_clause(std::unique_ptr<basic_condition> &&cond)
 {
-  data_.parts.push_back(std::make_unique<query_where_part>(std::move(cond)));
+  if (cond) {
+    data_->parts.push_back(std::make_unique<query_where_part>(std::move(cond)));
+  }
   return {connection_, schema_, data_};
 }
 
 query_group_by_intermediate query_from_intermediate::group_by(const column &col)
 {
-  data_.parts.push_back(std::make_unique<query_group_by_part>(col));
+  data_->parts.push_back(std::make_unique<query_group_by_part>(col));
   return {connection_, schema_, data_};
 }
 
 query_order_by_intermediate query_from_intermediate::order_by(const column &col)
 {
-  data_.parts.push_back(std::make_unique<query_order_by_part>(col));
+  data_->parts.push_back(std::make_unique<query_order_by_part>(col));
   return {connection_, schema_, data_};
 }
 
 query_group_by_intermediate query_where_intermediate::group_by(const column &col)
 {
-  data_.parts.push_back(std::make_unique<query_group_by_part>(col));
+  data_->parts.push_back(std::make_unique<query_group_by_part>(col));
   return {connection_, schema_, data_};
 }
 
 query_order_by_intermediate query_where_intermediate::order_by(const column &col)
 {
-  data_.parts.push_back(std::make_unique<query_order_by_part>(col));
-  return {connection_, schema_, data_};
-}
-
-query_join_intermediate query_on_intermediate::join_left(const table &t)
-{
-  data_.parts.push_back(std::make_unique<query_join_part>(t));
-  return {connection_, schema_, data_};
-}
-
-query_where_intermediate query_on_intermediate::where_clause(std::unique_ptr<basic_condition> &&cond)
-{
-  data_.parts.push_back(std::make_unique<query_where_part>(std::move(cond)));
-  return {connection_, schema_, data_};
-}
-
-query_group_by_intermediate query_on_intermediate::group_by(const column &col)
-{
-  data_.parts.push_back(std::make_unique<query_group_by_part>(col));
-  return {connection_, schema_, data_};
-}
-
-query_order_by_intermediate query_on_intermediate::order_by(const column &col)
-{
-  data_.parts.push_back(std::make_unique<query_order_by_part>(col));
+  data_->parts.push_back(std::make_unique<query_order_by_part>(col));
   return {connection_, schema_, data_};
 }
 
 query_on_intermediate query_join_intermediate::on_clause(std::unique_ptr<basic_condition> &&cond)
 {
-  data_.parts.push_back(std::make_unique<query_on_part>(std::move(cond)));
+  data_->parts.push_back(std::make_unique<query_on_part>(std::move(cond)));
   return {connection_, schema_, data_};
 }
 
 query_join_intermediate query_from_intermediate::join_left(const table &t)
 {
-  data_.parts.push_back(std::make_unique<query_join_part>(t));
+  data_->parts.push_back(std::make_unique<query_join_part>(t));
+  return {connection_, schema_, data_};
+}
+
+query_from_intermediate query_from_intermediate::join_left(join_data &data)
+{
+  data_->parts.push_back(std::make_unique<query_join_part>(data.join_table));
+  data_->parts.push_back(std::make_unique<query_on_part>(std::move(data.condition)));
+  return {connection_, schema_, data_};
+}
+
+query_from_intermediate query_from_intermediate::join_left(std::vector<join_data> &data_vector)
+{
+  for (auto &data : data_vector) {
+    data_->parts.push_back(std::make_unique<query_join_part>(data.join_table));
+    data_->parts.push_back(std::make_unique<query_on_part>(std::move(data.condition)));
+  }
   return {connection_, schema_, data_};
 }
 
 query_select_intermediate::query_select_intermediate(connection &db, const sql::schema &schema, const std::vector<column>& columns)
 : query_start_intermediate(db, schema)
 {
-  data_.parts.push_back(std::make_unique<query_select_part>(columns));
+  data_->parts.push_back(std::make_unique<query_select_part>(columns));
 }
 
 query_from_intermediate query_select_intermediate::from(const table& t)
 {
-  data_.parts.push_back(std::make_unique<query_from_part>(t));
+  data_->parts.push_back(std::make_unique<query_from_part>(t));
   return {connection_, schema_, data_};
 }
 
 query_insert_intermediate::query_insert_intermediate(connection &db, const sql::schema &schema)
 : query_start_intermediate(db, schema)
 {
-  data_.parts.push_back(std::make_unique<query_insert_part>());
+  data_->parts.push_back(std::make_unique<query_insert_part>());
 }
 
 query_into_intermediate query_insert_intermediate::into(const sql::table &table, std::initializer_list<column> column_names)
@@ -174,89 +168,89 @@ query_into_intermediate query_insert_intermediate::into(const sql::table &table,
 
 query_into_intermediate query_insert_intermediate::into(const table &table, std::vector<column> &&column_names)
 {
-  data_.parts.push_back(std::make_unique<query_into_part>(table, column_names));
+  data_->parts.push_back(std::make_unique<query_into_part>(table, column_names));
   return {connection_, schema_, data_};
 }
 
 query_into_intermediate query_insert_intermediate::into(const table &table)
 {
-  data_.parts.push_back(std::make_unique<query_into_part>(table, table.columns));
+  data_->parts.push_back(std::make_unique<query_into_part>(table, table.columns));
   return {connection_, schema_, data_};
 }
 
-size_t query_execute_finish::execute()
+size_t query_execute::execute()
 {
   query_compiler compiler(connection_.dialect());
-  return connection_.execute(compiler.compile(&data_).sql);
+  return connection_.execute(compiler.compile(data_.get()).sql);
 }
 
-statement query_execute_finish::prepare()
+statement query_execute::prepare()
 {
   query_compiler compiler(connection_.dialect());
-  return connection_.prepare(compiler.compile(&data_));
+  return connection_.prepare(compiler.compile(data_.get()));
 }
 
-query_context query_execute_finish::build() const
+query_context query_execute::build() const
 {
   query_compiler compiler(connection_.dialect());
-  return compiler.compile(&data_);
+  return compiler.compile(data_.get());
 }
 
-query_execute_finish query_into_intermediate::values(std::initializer_list<any_type> values)
+query_execute query_into_intermediate::values(std::initializer_list<any_type> values)
 {
   return this->values(std::vector<any_type>(values));
 }
 
-query_execute_finish query_into_intermediate::values(std::vector<any_type> &&values)
+query_execute query_into_intermediate::values(std::vector<any_type> &&values)
 {
-  data_.parts.push_back(std::make_unique<query_values_part>(std::move(values)));
+  data_->parts.push_back(std::make_unique<query_values_part>(std::move(values)));
   return {connection_, schema_, data_};
 }
 
 query_create_intermediate::query_create_intermediate(connection &db, const sql::schema &schema)
 : query_start_intermediate(db, schema) {
-  data_.parts.push_back(std::make_unique<query_create_part>());
+  data_->parts.push_back(std::make_unique<query_create_part>());
 }
 
-query_execute_finish query_create_intermediate::table(const sql::table &table, std::initializer_list<column_definition> columns)
+query_execute query_create_intermediate::table(const sql::table &table, std::initializer_list<column_definition> columns)
 {
   return this->table(table, std::vector<column_definition>{columns});
 }
 
-query_execute_finish query_create_intermediate::table(const sql::table &table, const std::vector<column_definition> &columns)
+query_execute query_create_intermediate::table(const sql::table &table, const std::vector<column_definition> &columns)
 {
-  data_.parts.push_back(std::make_unique<query_create_table_part>(table, columns));
+  data_->parts.push_back(std::make_unique<query_create_table_part>(table, columns));
   return {connection_, schema_, data_};
 }
 
 query_drop_intermediate::query_drop_intermediate(connection &db, const sql::schema &schema)
 : query_start_intermediate(db, schema)
 {
-  data_.parts.push_back(std::make_unique<query_drop_part>());
+  data_->parts.push_back(std::make_unique<query_drop_part>());
 }
 
-query_execute_finish query_drop_intermediate::table(const sql::table &table)
+query_execute query_drop_intermediate::table(const sql::table &table)
 {
-  data_.parts.push_back(std::make_unique<query_drop_table_part>(table));
+  data_->parts.push_back(std::make_unique<query_drop_table_part>(table));
   return {connection_, schema_, data_};
 }
 
 query_order_by_intermediate query_execute_where_intermediate::order_by(const column &col)
 {
-  data_.parts.push_back(std::make_unique<query_order_by_part>(col));
+  data_->parts.push_back(std::make_unique<query_order_by_part>(col));
   return {connection_, schema_, data_};
 }
 
 query_execute_where_intermediate query_set_intermediate::where_clause(std::unique_ptr<basic_condition> &&cond)
 {
-  data_.parts.push_back(std::make_unique<query_where_part>(std::move(cond)));
+  data_->parts.push_back(std::make_unique<query_where_part>(std::move(cond)));
   return {connection_, schema_, data_};
 }
 
 query_update_intermediate::query_update_intermediate(connection &db, const sql::schema &schema, const sql::table& table)
 : query_start_intermediate(db, schema)
 {
-  data_.parts.push_back(std::make_unique<query_update_part>(table));
+  data_->parts.push_back(std::make_unique<query_update_part>(table));
 }
 
 query_set_intermediate query_update_intermediate::set(std::initializer_list<key_value_pair> columns)
@@ -266,26 +260,26 @@ query_set_intermediate query_update_intermediate::set(std::initializer_list<key_
 
 query_set_intermediate query_update_intermediate::set(std::vector<key_value_pair> &&columns)
 {
-  data_.parts.push_back(std::make_unique<query_set_part>(std::move(columns)));
+  data_->parts.push_back(std::make_unique<query_set_part>(std::move(columns)));
   return {connection_, schema_, data_};
 
 }
 
 query_execute_where_intermediate query_delete_from_intermediate::where_clause(std::unique_ptr<basic_condition> &&cond)
 {
-  data_.parts.push_back(std::make_unique<query_where_part>(std::move(cond)));
+  data_->parts.push_back(std::make_unique<query_where_part>(std::move(cond)));
   return {connection_, schema_, data_};
 }
 
 query_delete_intermediate::query_delete_intermediate(connection &db, const sql::schema &schema)
 : query_start_intermediate(db, schema)
 {
-  data_.parts.push_back(std::make_unique<query_delete_part>());
+  data_->parts.push_back(std::make_unique<query_delete_part>());
 }
 
 query_delete_from_intermediate query_delete_intermediate::from(const sql::table &table)
 {
-  data_.parts.push_back(std::make_unique<query_delete_from_part>(table));
+  data_->parts.push_back(std::make_unique<query_delete_from_part>(table));
   return {connection_, schema_, data_};
 }
 

src/sql/query_result_impl.cpp

@@ -1,5 +1,6 @@
 #include "matador/sql/query_result_impl.hpp"
 #include "matador/sql/query_result_reader.hpp"
+#include "matador/sql/value.hpp"
 
 namespace matador::sql {
 
@@ -39,9 +40,9 @@ void query_result_impl::on_attribute(const char *id, std::string &value, const u
 }
 
 void
-query_result_impl::on_attribute(const char *id, any_type &value, data_type_t type, const utils::field_attributes &attr)
+query_result_impl::on_attribute(const char *id, value &val, const utils::field_attributes &attr)
 {
-  reader_->read_value(id, column_index_++, value, type, attr.size());
+  reader_->read_value(id, column_index_++, val, attr.size());
 }
 
 const std::vector<column_definition>& query_result_impl::prototype() const

src/sql/query_result_reader.cpp

@@ -1,5 +1,6 @@
 #include "matador/sql/query_result_reader.hpp"
 #include "matador/sql/to_value.hpp"
+#include "matador/sql/value.hpp"
 
 namespace matador::sql {
 
@@ -104,82 +105,82 @@ void query_result_reader::read_value(const char *id, size_t index, utils::blob &
 }
 
 template < typename Type >
-void convert(const char *valstr, sql::any_type &value)
+void convert(const char *valstr, value &val)
 {
-  Type val{};
-  sql::to_value(val, valstr);
-  value = val;
+  Type local_val{};
+  sql::to_value(local_val, valstr);
+  val = local_val;
 }
 
-void query_result_reader::read_value(const char *id, size_t index, any_type &value, data_type_t type, size_t size)
+void query_result_reader::read_value(const char *id, size_t index, value &val, size_t size)
 {
-  switch (type) {
+  switch (val.type()) {
     case sql::data_type_t::type_char:
-      convert<char>(column(index), value);
+      convert<char>(column(index), val);
       break;
     case sql::data_type_t::type_short:
-      convert<short>(column(index), value);
+      convert<short>(column(index), val);
       break;
     case sql::data_type_t::type_int:
-      convert<int>(column(index), value);
+      convert<int>(column(index), val);
       break;
     case sql::data_type_t::type_long:
-      convert<long>(column(index), value);
+      convert<long>(column(index), val);
       break;
     case sql::data_type_t::type_long_long:
-      convert<long long>(column(index), value);
+      convert<long long>(column(index), val);
       break;
     case sql::data_type_t::type_unsigned_char:
-      convert<unsigned char>(column(index), value);
+      convert<unsigned char>(column(index), val);
       break;
     case sql::data_type_t::type_unsigned_short:
-      convert<unsigned short>(column(index), value);
+      convert<unsigned short>(column(index), val);
       break;
     case sql::data_type_t::type_unsigned_int:
-      convert<unsigned int>(column(index), value);
+      convert<unsigned int>(column(index), val);
       break;
     case sql::data_type_t::type_unsigned_long:
-      convert<unsigned long>(column(index), value);
+      convert<unsigned long>(column(index), val);
       break;
     case sql::data_type_t::type_unsigned_long_long:
-      convert<unsigned long long>(column(index), value);
+      convert<unsigned long long>(column(index), val);
       break;
     case sql::data_type_t::type_float:
-      convert<float>(column(index), value);
+      convert<float>(column(index), val);
       break;
     case sql::data_type_t::type_double:
-      convert<double>(column(index), value);
+      convert<double>(column(index), val);
       break;
     case sql::data_type_t::type_bool: {
-      int val{};
-      sql::to_value(val, column(index));
-      value = val > 0;
+      int local_val{};
+      sql::to_value(local_val, column(index));
+      val = local_val > 0;
       break;
     }
     case sql::data_type_t::type_text:
     case sql::data_type_t::type_varchar: {
-      value = std::string{column(index)};
+      val = std::string{column(index)};
       break;
     }
     case sql::data_type_t::type_char_pointer: {
-      value = column(index);
+      val = column(index);
       break;
     }
     case sql::data_type_t::type_time:
     case sql::data_type_t::type_date: {
-      value = std::string{column(index)};
+      val = std::string{column(index)};
       break;
     }
     case sql::data_type_t::type_null: {
-      value = nullptr_t{};
+      val = nullptr_t{};
       break;
     }
     case sql::data_type_t::type_blob: {
-      value = utils::blob{};
+      val = utils::blob{};
       break;
     }
     case sql::data_type_t::type_unknown: {
-      value = std::string(column(index));
+      val = std::string(column(index));
       break;
     }
   }

src/sql/record.cpp

@@ -7,23 +7,27 @@
 namespace matador::sql {
 
 record::record(std::initializer_list<field> columns)
-: fields_(columns)
 {
-  init();
+  for (auto &&col :columns) {
+    const auto it = fields_by_name_.emplace(col.name(), field_index_pair {col, fields_.size()});
+    fields_.push_back(std::ref(it.first->second.first));
+  }
 }
 
 record::record(const std::vector<field> &columns)
-: fields_(columns)
 {
-  init();
+  for (auto &&col :columns) {
+    const auto it = fields_by_name_.emplace(col.name(), field_index_pair {col, fields_.size()});
+    fields_.push_back(std::ref(it.first->second.first));
+  }
 }
 
 record::record(const record &x)
-: fields_(x.fields_)
+: fields_by_name_(x.fields_by_name_)
 //, pk_index_(x.pk_index_)
 {
-  for (auto& col : fields_) {
-    add_to_map(col, col.index());
+  for (auto& col : fields_by_name_) {
+    fields_.push_back(std::ref(col.second.first));
   }
 }
 
@@ -33,16 +37,16 @@ record &record::operator=(const record &x)
     return *this;
   }
 
-  fields_ = x.fields_;
-  fields_by_name_.clear();
+  fields_by_name_ = x.fields_by_name_;
+  fields_.clear();
 //  pk_index_ = x.pk_index_;
-  for (auto& col : fields_) {
-    add_to_map(col, col.index());
+  for (auto& col : fields_by_name_) {
+    fields_.push_back(std::ref(col.second.first));
   }
   return *this;
 }
 
-const std::vector<field> &record::columns() const
+const std::vector<record::field_ref> &record::columns() const
 {
   return fields_;
 }
@@ -63,7 +67,9 @@ const std::vector<field> &record::columns() const
 
 const field &record::at(const column &col) const
 {
-  return fields_by_name_.at(col.name).first;
+  const auto &res = fields_by_name_.at(col.name);
+  const auto &f = res.first;
+  return f;
 }
 
 const field &record::at(size_t index) const
@@ -82,10 +88,10 @@ record::const_iterator record::find(const std::string &column_name) const {
   return it != fields_by_name_.end() ? fields_.begin() + it->second.second : fields_.end();
 }
 
-void record::append(field col)
+void record::append(const field &col)
 {
-  auto &ref = fields_.emplace_back(std::move(col));
-  add_to_map(ref, fields_.size() - 1);
+  const auto it = fields_by_name_.emplace(col.name(), field_index_pair {col, fields_.size()});
+  fields_.push_back(std::ref(it.first->second.first));
 }
 
 record::iterator record::begin()

src/sql/schema.cpp

@@ -8,6 +8,13 @@ namespace matador::sql {
 schema::schema(std::string name)
 : name_(std::move(name)) {}
 
+void schema::create(connection &c) {
+    for (const auto &ti : repository_) {
+        // ti.second.prototype
+
+    }
+}
+
 std::string schema::name() const
 {
   return name_;
@@ -15,7 +22,10 @@ std::string schema::name() const
 
 const table_info& schema::attach(const std::type_index ti, const table_info& table)
 {
-  return repository_.try_emplace(ti, table).first->second;
+
+  auto &ref = repository_.try_emplace(ti, table).first->second;
+  repository_by_name_.try_emplace(ref.name, std::ref(ref));
+  return ref;
 }
 
 std::optional<table_info> schema::info(std::type_index ti) const
@@ -27,6 +37,15 @@ std::optional<table_info> schema::info(std::type_index ti) const
   return it->second;
 }
 
+std::optional<table_info> schema::info(const std::string &name) const
+{
+  const auto it = repository_by_name_.find(name);
+  if (it == repository_by_name_.end()) {
+    return std::nullopt;
+  }
+  return it->second;
+}
+
 std::pair<std::string, std::string> schema::reference(const std::type_index &ti) const
 {
   const auto it = repository_.find(ti);

src/sql/session.cpp

@@ -89,11 +89,6 @@ bool session::table_exists(const std::string &table_name) const
   return c->exists(dialect_.default_schema_name(), table_name);
 }
 
-const schema& session::tables() const
-{
-  return *schema_;
-}
-
 const class dialect &session::dialect() const
 {
   return dialect_;
@@ -108,4 +103,15 @@ std::unique_ptr<query_result_impl> session::fetch(const std::string &sql) const
   return c->fetch(sql);
 }
 
+query_select session::build_select_query(connection_ptr<connection> &conn, entity_query_data &&data) const
+{
+  return conn->query(*schema_)
+    .select(data.columns)
+    .from(data.root_table_name)
+    .join_left(data.joins)
+    .where(std::move(data.where_clause))
+    .order_by({data.root_table_name, data.pk_column_})
+    .asc();
+}
+
 }

src/sql/value.cpp

@@ -0,0 +1,101 @@
+#include "matador/sql/value.hpp"
+
+#include <cstring>
+
+namespace matador::sql {
+namespace detail {
+
+size_t determine_size(const std::string &val)
+{
+  return val.size();
+}
+
+size_t determine_size(const char *val)
+{
+  return strlen(val);
+}
+
+size_t determine_size(const utils::blob &val)
+{
+  return val.size();
+}
+
+}
+value::value(data_type_t data_type, size_t size)
+: size_(size)
+, type_(data_type) {}
+
+value::value(value &&x) noexcept
+  : value_(std::move(x.value_))
+  , type_(x.type_)
+{
+  x.value_ = nullptr;
+  x.type_ = data_type_t::type_unknown;
+}
+
+value &value::operator=(value &&x) noexcept
+{
+  value_ = std::move(x.value_);
+  type_ = x.type_;
+  x.value_ = nullptr;
+  x.type_ = data_type_t::type_unknown;
+
+  return *this;
+}
+
+std::string value::str() const
+{
+  return as<std::string>().value();
+}
+
+size_t value::size() const
+{
+  return size_;
+}
+
+data_type_t value::type() const
+{
+  return type_;
+}
+
+bool value::is_integer() const
+{
+  return type_ >= data_type_t::type_char && type_ <= data_type_t::type_unsigned_long_long;
+}
+
+bool value::is_floating_point() const
+{
+  return type_ == data_type_t::type_float || type_ == data_type_t::type_double;
+}
+
+bool value::is_bool() const
+{
+  return type_ == data_type_t::type_bool;
+}
+
+bool value::is_string() const
+{
+  return type_ == data_type_t::type_text;
+}
+
+bool value::is_varchar() const
+{
+  return type_ == data_type_t::type_varchar || type_ == data_type_t::type_char_pointer;
+}
+
+bool value::is_blob() const
+{
+  return type_ == data_type_t::type_blob;
+}
+
+bool value::is_null() const
+{
+  return type_ == data_type_t::type_null;
+}
+
+bool value::is_unknown() const
+{
+  return type_ == data_type_t::type_unknown;
+}
+
+}

test/CMakeLists.txt

@@ -3,7 +3,7 @@ Include(FetchContent)
 FetchContent_Declare(
         Catch2
         GIT_REPOSITORY https://github.com/catchorg/Catch2.git
-        GIT_TAG        v3.4.0 # or a later release
+        GIT_TAG        v3.5.4 # or a later release
 )
 
 FetchContent_MakeAvailable(Catch2)
@@ -19,7 +19,7 @@ add_executable(tests
   BackendProviderTest.cpp
   models/product.hpp
   models/order.hpp
-  models/order_details.h
+  models/order_details.hpp
   ColumnDefinitionGeneratorTest.cpp
   ColumnGeneratorTest.cpp
   ValueGeneratorTest.cpp
@@ -34,13 +34,16 @@ add_executable(tests
   models/location.hpp
   models/optional.hpp
   ConvertTest.cpp
-  DummyConnection.hpp
-  DummyConnection.cpp
   EntityQueryBuilderTest.cpp
   models/author.hpp
   models/book.hpp
   FieldTest.cpp
-  models/recipe.hpp)
+  models/recipe.hpp
+  ValueTest.cpp
+  ResultTest.cpp
+  utils/auto_reset_event.hpp
+  utils/auto_reset_event.cpp
+  models/student.hpp)
 
 target_link_libraries(tests PRIVATE
   Catch2::Catch2WithMain

test/ColumnDefinitionGeneratorTest.cpp

@@ -9,7 +9,7 @@
 using namespace matador::sql;
 using namespace matador::utils;
 
-TEST_CASE("Generate columns from object", "[column generator]") {
+TEST_CASE("Generate column definitions from object", "[column][definition][generator]") {
   schema repo("main");
 
   auto columns = column_definition_generator::generate<matador::test::product>(repo);

test/ConnectionPoolTest.cpp

@@ -1,15 +1,17 @@
 #include <catch2/catch_test_macros.hpp>
 
-#include <matador/sql/connection_pool.hpp>
+#include "matador/sql/connection_pool.hpp"
+#include "matador/sql/noop_connection.hpp"
 
-#include "DummyConnection.hpp"
+#include "utils/auto_reset_event.hpp"
 
 using namespace matador::sql;
+using namespace matador::test::utils;
 
 TEST_CASE("Create connection pool", "[connection pool]") {
-  using pool_t = connection_pool<matador::test::DummyConnection>;
+  using pool_t = connection_pool<noop_connection>;
 
-  pool_t pool("sqlite://sqlite.db", 4);
+  pool_t pool("noop://noop.db", 4);
 
   REQUIRE(pool.size() == 4);
   REQUIRE(pool.idle() == 4);
@@ -51,9 +53,9 @@ TEST_CASE("Create connection pool", "[connection pool]") {
 }
 
 TEST_CASE("Acquire connection by id", "[connection pool]") {
-  using pool_t = connection_pool<matador::test::DummyConnection>;
+  using pool_t = connection_pool<noop_connection>;
 
-  pool_t pool("sqlite://sqlite.db", 4);
+  pool_t pool("noop://noop.db", 4);
 
   REQUIRE(pool.size() == 4);
   REQUIRE(pool.idle() == 4);
@@ -79,3 +81,74 @@ TEST_CASE("Acquire connection by id", "[connection pool]") {
   REQUIRE(same_ptr.valid());
   REQUIRE(same_ptr.id() == connection_id);
 }
+
+TEST_CASE("Try acquire connection", "[connection pool][try acquire]") {
+  using pool_t = connection_pool<noop_connection>;
+
+  pool_t pool("noop://noop.db", 1);
+
+  REQUIRE(pool.size() == 1);
+  REQUIRE(pool.idle() == 1);
+  REQUIRE(pool.inuse() == 0);
+
+  auto ptr = pool.try_acquire();
+  REQUIRE(ptr.valid());
+  REQUIRE(ptr.id());
+  REQUIRE(ptr.id().value() > 0);
+  REQUIRE(ptr->is_open());
+  REQUIRE(pool.size() == 1);
+  REQUIRE(pool.idle() == 0);
+  REQUIRE(pool.inuse() == 1);
+
+  auto ptr2 = pool.try_acquire();
+  REQUIRE(!ptr2.valid());
+
+  pool.release(ptr);
+  REQUIRE(!ptr.valid());
+  REQUIRE(pool.size() == 1);
+  REQUIRE(pool.idle() == 1);
+  REQUIRE(pool.inuse() == 0);
+
+  ptr2 = pool.try_acquire();
+  REQUIRE(ptr2.valid());
+  REQUIRE(ptr2.id());
+  REQUIRE(ptr2.id().value() > 0);
+  REQUIRE(ptr2->is_open());
+  REQUIRE(pool.size() == 1);
+  REQUIRE(pool.idle() == 0);
+  REQUIRE(pool.inuse() == 1);
+
+  pool.release(ptr2);
+
+  auto_reset_event reset_main_event;
+  auto_reset_event reset_thread_event;
+
+  std::thread t([&reset_main_event, &reset_thread_event, &pool]() {
+    auto c1 = pool.acquire();
+    REQUIRE(c1.valid());
+    REQUIRE(c1.id());
+    REQUIRE(c1.id().value() > 0);
+
+    reset_main_event.set();
+
+    reset_thread_event.wait_one();
+
+    pool.release(c1);
+    REQUIRE(!c1.valid());
+    reset_main_event.set();
+  });
+  reset_main_event.wait_one();
+
+  ptr2 = pool.try_acquire();
+  REQUIRE(!ptr2.valid());
+
+  reset_thread_event.set();
+
+  reset_main_event.wait_one();
+  ptr2 = pool.try_acquire();
+  REQUIRE(ptr2.valid());
+  REQUIRE(ptr2.id());
+  REQUIRE(ptr2.id().value() > 0);
+
+  t.join();
+}

test/DummyConnection.cpp

@@ -1,28 +0,0 @@
-#include "DummyConnection.hpp"
-
-namespace matador::test {
-
-DummyConnection::DummyConnection(sql::connection_info info)
-: connection_info_(std::move(info))
-{}
-
-DummyConnection::DummyConnection(const std::string &dns)
-: DummyConnection(sql::connection_info::parse(dns))
-{}
-
-void DummyConnection::open()
-{
-  is_open_ = true;
-}
-
-void DummyConnection::close()
-{
-  is_open_ = false;
-}
-
-bool DummyConnection::is_open() const
-{
-  return is_open_;
-}
-
-}

test/DummyConnection.hpp

@@ -1,30 +0,0 @@
-#ifndef QUERY_DUMMYCONNECTION_HPP
-#define QUERY_DUMMYCONNECTION_HPP
-
-#include "matador/sql/connection_info.hpp"
-
-namespace matador::test {
-
-class DummyConnection
-{
-public:
-  explicit DummyConnection(sql::connection_info info);
-  explicit DummyConnection(const std::string& dns);
-  DummyConnection(const DummyConnection &x) = default;
-  DummyConnection& operator=(const DummyConnection &x) = default;
-  DummyConnection(DummyConnection &&x) noexcept = default;
-  DummyConnection& operator=(DummyConnection &&x) noexcept = default;
-  ~DummyConnection() = default;
-
-  void open();
-  void close();
-  [[nodiscard]] bool is_open() const;
-
-private:
-  sql::connection_info connection_info_;
-
-  bool is_open_{};
-};
-
-}
-#endif //QUERY_DUMMYCONNECTION_HPP

test/EntityQueryBuilderTest.cpp

@@ -3,31 +3,256 @@
 #include <matador/sql/connection.hpp>
 #include <matador/sql/entity_query_builder.hpp>
 
+#include "models/airplane.hpp"
 #include "models/author.hpp"
 #include "models/book.hpp"
-
-struct schiff {
-  std::string id;
-
-  template<typename Operator>
-  void process(Operator &op)
-  {
-    namespace field = matador::utils::access;
-    field::primary_key(op, "id", id, 255);
-  }
-};
+#include "models/flight.hpp"
+#include "models/recipe.hpp"
+#include "models/order.hpp"
+#include "models/student.hpp"
 
 using namespace matador::sql;
 
-TEST_CASE("Create sql query for entity", "[query][entity][builder]") {
-  connection noop("noop://noop.db");
+TEST_CASE("Create sql query data for entity with eager has one", "[query][entity][builder]") {
+  using namespace matador::test;
+  connection db("noop://noop.db");
   schema scm("noop");
-  scm.attach<matador::test::author>("authors");
-  scm.attach<matador::test::book>("books");
+  scm.attach<airplane>("airplanes");
+  scm.attach<flight>("flights");
 
-  entity_query_builder eqb(17, scm);
+  entity_query_builder eqb(scm);
 
-  auto context = eqb.build<matador::test::book>(noop);
-  std::cout << "SQL: " << context.value().sql << "\n";
-  context = eqb.build<schiff>(noop);
+  auto data = eqb.build<flight>(17);
+
+  REQUIRE(data.is_ok());
+  REQUIRE(data->root_table_name == "flights");
+  REQUIRE(data->joins.size() == 1);
+  const std::vector<column> expected_columns {
+    { "flights", "id", "c01" },
+    { "airplanes", "id", "c02" },
+    { "airplanes", "brand", "c03" },
+    { "airplanes", "model", "c04" },
+    { "flights", "pilot_name", "c05" },
+  };
+  REQUIRE(data->columns.size() == expected_columns.size());
+  for (size_t i = 0; i != expected_columns.size(); ++i) {
+    REQUIRE(expected_columns[i].equals(data->columns[i]));
+  }
+
+  std::vector<std::pair<std::string, std::string>> expected_join_data {
+    { "airplanes", R"("flights"."airplane_id" = "airplanes"."id")"}
+  };
+
+  query_context qc;
+  size_t index{0};
+  for (const auto &jd : data->joins) {
+    REQUIRE(jd.join_table.name == expected_join_data[index].first);
+    REQUIRE(jd.condition->evaluate(db.dialect(), qc) == expected_join_data[index].second);
+    ++index;
+  }
+
+  REQUIRE(data->where_clause);
+  auto cond = data->where_clause->evaluate(db.dialect(), qc);
+  REQUIRE(cond == R"("flights"."id" = 17)");
+}
+
+TEST_CASE("Create sql query data for entity with eager belongs to", "[query][entity][builder]") {
+  using namespace matador::test;
+  connection db("noop://noop.db");
+  schema scm("noop");
+  scm.attach<author>("authors");
+  scm.attach<book>("books");
+
+  entity_query_builder eqb(scm);
+
+  auto data = eqb.build<book>(17);
+
+  REQUIRE(data.is_ok());
+  REQUIRE(data->root_table_name == "books");
+  REQUIRE(data->joins.size() == 1);
+  const std::vector<column> expected_columns {
+    { "books", "id", "c01" },
+    { "books", "title", "c02" },
+    { "authors", "id", "c03" },
+    { "authors", "first_name", "c04" },
+    { "authors", "last_name", "c05" },
+    { "authors", "date_of_birth", "c06" },
+    { "authors", "year_of_birth", "c07" },
+    { "authors", "distinguished", "c08" },
+    { "books", "published_in", "c09" }
+  };
+  REQUIRE(data->columns.size() == expected_columns.size());
+  for (size_t i = 0; i != expected_columns.size(); ++i) {
+    REQUIRE(expected_columns[i].equals(data->columns[i]));
+  }
+
+  std::vector<std::pair<std::string, std::string>> expected_join_data {
+    { "authors", R"("books"."author_id" = "authors"."id")"}
+  };
+
+  query_context qc;
+  size_t index{0};
+  for (const auto &jd : data->joins) {
+    REQUIRE(jd.join_table.name == expected_join_data[index].first);
+    REQUIRE(jd.condition->evaluate(db.dialect(), qc) == expected_join_data[index].second);
+    ++index;
+  }
+
+  REQUIRE(data->where_clause);
+  auto cond = data->where_clause->evaluate(db.dialect(), qc);
+  REQUIRE(cond == R"("books"."id" = 17)");
+
+  auto q = db.query(scm)
+    .select(data->columns)
+    .from(data->root_table_name);
+
+  for (auto &jd : data->joins) {
+    q.join_left(jd.join_table)
+      .on(std::move(jd.condition));
+  }
+  auto context = q
+    .where(std::move(data->where_clause))
+    .build();
+}
+
+TEST_CASE("Create sql query data for entity with eager has many belongs to", "[query][entity][builder]") {
+  using namespace matador::test;
+  connection db("noop://noop.db");
+  schema scm("noop");
+  scm.attach<product>("products");
+  scm.attach<order_details>("order_details");
+  scm.attach<order>("orders");
+
+  entity_query_builder eqb(scm);
+
+  auto data = eqb.build<order>(17);
+
+  REQUIRE(data.is_ok());
+  REQUIRE(data->root_table_name == "orders");
+  REQUIRE(data->joins.size() == 1);
+  const std::vector<column> expected_columns = {
+    { "orders", "order_id", "c01" },
+    { "orders", "order_date", "c02" },
+    { "orders", "required_date", "c03" },
+    { "orders", "shipped_date", "c04" },
+    { "orders", "ship_via", "c05" },
+    { "orders", "freight", "c06" },
+    { "orders", "ship_name", "c07" },
+    { "orders", "ship_address", "c08" },
+    { "orders", "ship_city", "c09" },
+    { "orders", "ship_region", "c10" },
+    { "orders", "ship_postal_code", "c11" },
+    { "orders", "ship_country", "c12" },
+    { "order_details", "order_details_id", "c13" },
+    { "order_details", "order_id", "c14" },
+    { "order_details", "product_id", "c15" }
+  };
+  REQUIRE(data->columns.size() == expected_columns.size());
+  for (size_t i = 0; i != expected_columns.size(); ++i) {
+    REQUIRE(expected_columns[i].equals(data->columns[i]));
+  }
+
+  std::vector<std::pair<std::string, std::string>> expected_join_data {
+    { "order_details", R"("orders"."order_id" = "order_details"."order_id")"}
+  };
+
+  query_context qc;
+  size_t index{0};
+  for (const auto &jd : data->joins) {
+    REQUIRE(jd.join_table.name == expected_join_data[index].first);
+    REQUIRE(jd.condition->evaluate(db.dialect(), qc) == expected_join_data[index].second);
+    ++index;
+  }
+
+  REQUIRE(data->where_clause);
+  auto cond = data->where_clause->evaluate(db.dialect(), qc);
+  REQUIRE(cond == R"("orders"."order_id" = 17)");
+}
+
+TEST_CASE("Create sql query data for entity with eager many to many", "[query][entity][builder]") {
+  using namespace matador::test;
+  connection db("noop://noop.db");
+  schema scm("noop");
+  scm.attach<recipe>("recipes");
+  scm.attach<ingredient>("ingredients");
+  scm.attach<recipe_ingredient>("recipe_ingredients");
+
+  entity_query_builder eqb(scm);
+
+  auto data = eqb.build<ingredient>(17);
+
+  REQUIRE(data.is_ok());
+  REQUIRE(data->root_table_name == "ingredients");
+  REQUIRE(data->joins.size() == 2);
+  const std::vector<column> expected_columns {
+    { "ingredients", "id", "c01" },
+    { "ingredients", "name", "c02" },
+    { "recipes", "id", "c03" },
+    { "recipes", "name", "c04" }
+  };
+  REQUIRE(data->columns.size() == expected_columns.size());
+  for (size_t i = 0; i != expected_columns.size(); ++i) {
+    REQUIRE(expected_columns[i].equals(data->columns[i]));
+  }
+
+  std::vector<std::pair<std::string, std::string>> expected_join_data {
+    { "recipe_ingredients", R"("ingredients"."id" = "recipe_ingredients"."ingredient_id")"},
+    { "recipes", R"("recipe_ingredients"."recipe_id" = "recipes"."id")"}
+  };
+
+  query_context qc;
+  size_t index{0};
+  for (const auto &jd : data->joins) {
+    REQUIRE(jd.join_table.name == expected_join_data[index].first);
+    REQUIRE(jd.condition->evaluate(db.dialect(), qc) == expected_join_data[index].second);
+    ++index;
+  }
+
+  REQUIRE(data->where_clause);
+  auto cond = data->where_clause->evaluate(db.dialect(), qc);
+  REQUIRE(cond == R"("ingredients"."id" = 17)");
+}
+
+TEST_CASE("Create sql query data for entity with eager many to many (inverse part)", "[query][entity][builder]") {
+  using namespace matador::test;
+  connection db("noop://noop.db");
+  schema scm("noop");
+  scm.attach<student>("students");
+  scm.attach<course>("courses");
+  scm.attach<student_course>("student_courses");
+
+  entity_query_builder eqb(scm);
+
+  auto data = eqb.build<course>(17);
+
+  REQUIRE(data.is_ok());
+  REQUIRE(data->root_table_name == "courses");
+  REQUIRE(data->joins.size() == 2);
+  const std::vector<column> expected_columns {
+    { "courses", "id", "c01" },
+    { "courses", "title", "c02" },
+    { "students", "id", "c03" },
+    { "students", "name", "c04" }
+  };
+  REQUIRE(data->columns.size() == expected_columns.size());
+  for (size_t i = 0; i != expected_columns.size(); ++i) {
+    REQUIRE(expected_columns[i].equals(data->columns[i]));
+  }
+
+  std::vector<std::pair<std::string, std::string>> expected_join_data {
+    { "student_courses", R"("courses"."id" = "student_courses"."course_id")"},
+    { "students", R"("student_courses"."student_id" = "students"."id")"}
+  };
+
+  query_context qc;
+  size_t index{0};
+  for (const auto &jd : data->joins) {
+    REQUIRE(jd.join_table.name == expected_join_data[index].first);
+    REQUIRE(jd.condition->evaluate(db.dialect(), qc) == expected_join_data[index].second);
+    ++index;
+  }
+
+  REQUIRE(data->where_clause);
+  auto cond = data->where_clause->evaluate(db.dialect(), qc);
+  REQUIRE(cond == R"("courses"."id" = 17)");
 }

test/FieldTest.cpp

@@ -9,8 +9,8 @@ TEST_CASE("Field test", "[field]") {
 
   REQUIRE(f.name() == "name");
   REQUIRE(f.index() == -1);
-  REQUIRE(f.is_unknown());
-  REQUIRE(!f.is_null());
+  REQUIRE(!f.is_unknown());
+  REQUIRE(f.is_null());
   REQUIRE(!f.is_integer());
   REQUIRE(!f.is_floating_point());
   REQUIRE(!f.is_blob());

test/ResultTest.cpp

@@ -0,0 +1,95 @@
+#include <catch2/catch_test_macros.hpp>
+
+#include "matador/utils/result.hpp"
+
+namespace matador::test {
+
+enum class math_error : int32_t {
+  DIVISION_BY_ZERO = 1
+};
+
+utils::result<float, math_error>divide(int x, int y) {
+  if (y == 0) {
+    return utils::error(math_error::DIVISION_BY_ZERO);
+  }
+
+  return utils::ok(float(x) / y);
+}
+
+utils::result<float, math_error>multiply(int x, int y) {
+  return utils::ok(float(x) * y);
+}
+
+utils::result<float, math_error>plus(int x, int y) {
+  return utils::ok(float(x) + y);
+}
+
+utils::result<float, std::string>error_to_string(math_error err) {
+  switch (err) {
+    case math_error::DIVISION_BY_ZERO:
+      return utils::error(std::string("division by zero error"));
+    default:
+      return utils::error(std::string("unknown error"));
+  }
+}
+
+}
+
+using namespace matador;
+
+TEST_CASE("Result tests", "[result]") {
+  auto res = test::divide(4, 2);
+  REQUIRE(res);
+  REQUIRE(res.is_ok());
+  REQUIRE(!res.is_error());
+
+  REQUIRE((res.value() == 2.0));
+
+  REQUIRE_THROWS(res.err());
+
+  res = test::divide(4, 0);
+  REQUIRE(!res);
+  REQUIRE(!res.is_ok());
+  REQUIRE(res.is_error());
+
+  REQUIRE((res.err() == test::math_error::DIVISION_BY_ZERO));
+
+  res = test::divide(4, 2)
+    .and_then([](const auto &val) { return test::multiply(val, 5); })
+    .and_then([](const auto &val) { return test::plus(val, 10); });
+
+  REQUIRE(res);
+  REQUIRE(res.is_ok());
+  REQUIRE(!res.is_error());
+  REQUIRE((res.value() == 20.0));
+
+  res = test::divide(4, 0)
+    .and_then([](const auto &val) {
+      return test::multiply(val, 5);
+    });
+
+  REQUIRE(!res);
+  REQUIRE(!res.is_ok());
+  REQUIRE(res.is_error());
+  REQUIRE((res.err() == test::math_error::DIVISION_BY_ZERO));
+
+  auto res2 = test::divide(4, 0)
+    .or_else([](const auto &err) {
+      return test::error_to_string(err);
+    });
+
+  REQUIRE(!res2);
+  REQUIRE(!res2.is_ok());
+  REQUIRE(res2.is_error());
+  REQUIRE((res2.err() == "division by zero error"));
+
+  res = test::divide(4, 2)
+    .and_then([](const auto &val) { return test::multiply(val, 5); })
+    .transform([](const auto &val) { return val + 10; });
+
+  REQUIRE(res);
+  REQUIRE(res.is_ok());
+  REQUIRE(!res.is_error());
+  REQUIRE((res.value() == 20.0));
+
+}

test/ValueTest.cpp

@@ -0,0 +1,70 @@
+#include <catch2/catch_test_macros.hpp>
+
+#include "matador/sql/value.hpp"
+#include "matador/utils/types.hpp"
+
+using namespace matador::sql;
+
+TEST_CASE("Test value class", "[value]") {
+  value v;
+
+  REQUIRE(v.is_unknown());
+  REQUIRE(v.type() == data_type_t::type_unknown);
+  REQUIRE(v.size() == 0);
+
+  v = 7;
+
+  REQUIRE(v.is_integer());
+  REQUIRE(v.type() == data_type_t::type_int);
+  REQUIRE(v.size() == 0);
+  REQUIRE(v.as<int>() == 7);
+  REQUIRE(v.as<long>() == 7);
+
+  v = "test";
+
+  REQUIRE(v.is_varchar());
+  REQUIRE(v.type() == data_type_t::type_char_pointer);
+  REQUIRE(v.size() == 4);
+
+  v = std::string{"hello"};
+
+  REQUIRE(v.is_varchar());
+  REQUIRE(v.type() == data_type_t::type_varchar);
+  REQUIRE(v.size() == 5);
+
+  v = 4.5;
+
+  REQUIRE(v.is_floating_point());
+  REQUIRE(v.type() == data_type_t::type_double);
+  REQUIRE(v.size() == 0);
+
+  v = 6.7f;
+
+  REQUIRE(v.is_floating_point());
+  REQUIRE(v.type() == data_type_t::type_float);
+  REQUIRE(v.size() == 0);
+
+  v = std::string();
+
+  REQUIRE(v.is_string());
+  REQUIRE(v.type() == data_type_t::type_text);
+  REQUIRE(v.size() == 0);
+
+  v = true;
+
+  REQUIRE(v.is_bool());
+  REQUIRE(v.type() == data_type_t::type_bool);
+  REQUIRE(v.size() == 0);
+
+  v = nullptr;
+
+  REQUIRE(v.is_null());
+  REQUIRE(v.type() == data_type_t::type_null);
+  REQUIRE(v.size() == 0);
+
+  v = matador::utils::blob{ 1, 2, 3, 4 };
+
+  REQUIRE(v.is_blob());
+  REQUIRE(v.type() == data_type_t::type_blob);
+  REQUIRE(v.size() == 4);
+}

test/models/author.hpp

@@ -3,10 +3,14 @@
 
 #include "matador/utils/access.hpp"
 
+#include "matador/sql/entity.hpp"
+
 #include <string>
 
 namespace matador::test {
 
+struct book;
+
 struct author
 {
   unsigned long id{};
@@ -15,6 +19,7 @@ struct author
   std::string date_of_birth;
   unsigned short year_of_birth{};
   bool distinguished{false};
+  std::vector<matador::sql::entity<book>> books;
 
   template<typename Operator>
   void process(Operator &op)
@@ -26,6 +31,7 @@ struct author
     field::attribute(op, "date_of_birth", date_of_birth, 31);
     field::attribute(op, "year_of_birth", year_of_birth);
     field::attribute(op, "distinguished", distinguished);
+    field::has_many(op, books, "author_id", utils::fetch_type::LAZY);
   }
 };
 

test/models/book.hpp

@@ -25,7 +25,7 @@ struct book
     namespace field = matador::utils::access;
     field::primary_key(op, "id", id);
     field::attribute(op, "title", title, 511);
-    field::has_one(op, "author_id", book_author, utils::fetch_type::EAGER);
+    field::belongs_to(op, "author_id", book_author, utils::fetch_type::EAGER);
     field::attribute(op, "published_in", published_in);
   }
 };

test/models/order.hpp

@@ -1,7 +1,7 @@
 #ifndef QUERY_ORDER_HPP
 #define QUERY_ORDER_HPP
 
-#include "order_details.h"
+#include "order_details.hpp"
 
 #include "matador/utils/access.hpp"
 
@@ -42,7 +42,7 @@ struct order
     field::attribute(op, "ship_region", ship_region, 255);
     field::attribute(op, "ship_postal_code", ship_postal_code, 255);
     field::attribute(op, "ship_country", ship_country, 255);
-    field::has_many(op, "order_details", order_details_, utils::fetch_type::EAGER);
+    field::has_many(op, order_details_, "order_id", utils::fetch_type::EAGER);
   }
 };
 

test/models/order_details.hpp

@@ -1,5 +1,5 @@
-#ifndef QUERY_ORDER_DETAILS_H
-#define QUERY_ORDER_DETAILS_H
+#ifndef QUERY_ORDER_DETAILS_HPP
+#define QUERY_ORDER_DETAILS_HPP
 
 #include "product.hpp"
 
@@ -25,4 +25,4 @@ struct order_details
 };
 
 }
-#endif //QUERY_ORDER_DETAILS_H
+#endif //QUERY_ORDER_DETAILS_HPP

test/models/recipe.hpp

@@ -6,21 +6,25 @@
 #include "matador/utils/foreign_attributes.hpp"
 
 #include "matador/sql/entity.hpp"
+#include "matador/sql/has_many_to_many_relation.hpp"
 
 #include <string>
 
 namespace matador::test {
 
+struct recipe;
 struct ingredient
 {
   unsigned long id{};
   std::string name;
+  std::vector<matador::sql::entity<recipe>> recipes;
 
   template<class Operator>
   void process(Operator &op) {
     namespace field = matador::utils::access;
     field::primary_key(op, "id", id);
     field::attribute(op, "name", name, 255);
+    field::has_many_to_many(op, "recipe_ingredients", recipes, "ingredient_id", "recipe_id", utils::fetch_type::EAGER);
   }
 };
 
@@ -28,26 +32,22 @@ struct recipe
 {
   unsigned long id{};
   std::string name;
+  std::vector<matador::sql::entity<ingredient>> ingredients;
 
   template<class Operator>
   void process(Operator &op) {
     namespace field = matador::utils::access;
     field::primary_key(op, "id", id);
     field::attribute(op, "name", name, 255);
+    field::has_many_to_many(op, "recipe_ingredients", ingredients, utils::fetch_type::LAZY);
   }
 };
 
-struct recipe_ingredient
+class recipe_ingredient : public sql::has_many_to_many_relation<recipe, ingredient>
 {
-  sql::entity<recipe> recipe_;
-  sql::entity<ingredient> ingredient_;
-
-  template<class Operator>
-  void process(Operator &op) {
-    namespace field = matador::utils::access;
-    field::belongs_to(op, "recipe_id", recipe_, utils::default_foreign_attributes);
-    field::belongs_to(op, "ingredient_id", ingredient_, utils::default_foreign_attributes);
-  }
+public:
+  recipe_ingredient()
+  : has_many_to_many_relation("recipe_id", "ingredient_id") {}
 };
 
 }

test/models/student.hpp

@@ -0,0 +1,68 @@
+#ifndef QUERY_STUDENT_HPP
+#define QUERY_STUDENT_HPP
+
+#include "matador/utils/access.hpp"
+#include "matador/utils/foreign_attributes.hpp"
+
+#include "matador/sql/entity.hpp"
+#include "matador/sql/has_many_to_many_relation.hpp"
+
+#include <utility>
+#include <vector>
+
+namespace matador::test {
+
+class course;
+
+struct student
+{
+  unsigned long id{};
+  std::string name;
+  std::vector<matador::sql::entity<course>> courses;
+
+  student() = default;
+  explicit student(unsigned long id, std::string name)
+    : id(id)
+    , name(std::move(name)) {}
+
+  template < class Operator >
+  void process(Operator &op)
+  {
+    namespace field = matador::utils::access;
+    field::primary_key(op, "id", id);
+    field::attribute(op, "name", name, 255);
+    field::has_many_to_many(op, "student_courses", courses, "student_id", "course_id", utils::fetch_type::LAZY);
+  }
+
+};
+
+struct course
+{
+  unsigned long id{};
+  std::string title;
+  std::vector<matador::sql::entity<student>> students;
+
+  course() = default;
+  explicit course(unsigned long id, std::string title)
+  : id(id)
+  , title(std::move(title)) {}
+
+  template < class Operator >
+  void process(Operator &op)
+  {
+    namespace field = matador::utils::access;
+    field::primary_key(op, "id", id);
+    field::attribute(op, "title", title, 255);
+    field::has_many_to_many(op, "student_courses", students, utils::fetch_type::EAGER);
+  }
+};
+
+class student_course : public sql::has_many_to_many_relation<student, course>
+{
+public:
+  student_course()
+    : has_many_to_many_relation("student_id", "course_id") {}
+};
+
+}
+#endif //QUERY_STUDENT_HPP

test/utils/auto_reset_event.cpp

@@ -0,0 +1,26 @@
+#include "auto_reset_event.hpp"
+
+namespace matador::test::utils {
+
+auto_reset_event::auto_reset_event() : state(false) {}
+
+void auto_reset_event::wait_one()
+{
+  std::unique_lock<std::mutex> lock(sync);
+  underlying.wait(lock, [this](){return state.load();});
+  state = false;
+}
+
+void auto_reset_event::set()
+{
+  std::unique_lock<std::mutex> lock(sync);
+  state = true;
+  underlying.notify_one();
+}
+
+void auto_reset_event::reset()
+{
+  std::unique_lock<std::mutex> lock(sync);
+  state = false;
+}
+}

test/utils/auto_reset_event.hpp

@@ -0,0 +1,29 @@
+#ifndef QUERY_AUTO_RESET_EVENT_HPP
+#define QUERY_AUTO_RESET_EVENT_HPP
+
+#include <atomic>
+#include <condition_variable>
+#include <mutex>
+
+namespace matador::test::utils {
+
+class auto_reset_event
+{
+public:
+  auto_reset_event();
+  auto_reset_event(const auto_reset_event& other) = delete;
+
+  void wait_one();
+  void set();
+  void reset();
+
+private:
+  std::condition_variable underlying;
+  std::mutex sync;
+  std::atomic<bool> state;
+};
+
+
+}
+
+#endif //QUERY_AUTO_RESET_EVENT_HPP