has many to many relation inserts

include/matador/query/insert_query_builder.hpp

@@ -59,6 +59,28 @@ struct pk_unset_checker {
   static void on_has_many_to_many(const char * /*id*/, C &, const utils::foreign_attributes & ) {}
 };
 
+struct pk_value_extractor {
+  std::uint64_t value{0};
+
+  template<class V>
+  void on_primary_key(const char * /*id*/, V &pk, const utils::primary_key_attribute & = utils::default_pk_attributes) {
+    value = static_cast<std::uint64_t>(pk);
+  }
+  static void on_revision(const char * /*id*/, uint64_t & /*rev*/) {}
+  template<typename T>
+  static void on_attribute(const char * /*id*/, T &, const utils::field_attributes & = utils::null_attributes) {}
+  template<class P>
+  static void on_belongs_to(const char * /*id*/, P &, const utils::foreign_attributes & ) {}
+  template<class P>
+  static void on_has_one(const char * /*id*/, P &, const utils::foreign_attributes & ) {}
+  template<class C>
+  static void on_has_many(const char * /*id*/, C &, const char * /*join_column*/, const utils::foreign_attributes & ) {}
+  template<class C>
+  static void on_has_many_to_many(const char * /*id*/, C &, const char * /*join_column*/, const char * /*inverse_join_column*/, const utils::foreign_attributes & ) {}
+  template<class C>
+  static void on_has_many_to_many(const char * /*id*/, C &, const utils::foreign_attributes & ) {}
+};
+
 class insert_query_builder {
 public:
   using step_query_t = std::variant<executable_query, fetchable_query>;
@@ -91,6 +113,12 @@ public:
 
     build_for(ptr);
 
+    // relation inserts must run after all entity inserts were collected
+    for (auto &s : relation_steps_) {
+      steps_.push_back(std::move(s));
+    }
+    relation_steps_.clear();
+
     return utils::ok(steps_);
   }
 
@@ -110,10 +138,53 @@ public:
   void on_has_one(const char * /*id*/, Pointer &obj, const utils::foreign_attributes &attr) {
     on_foreign_object(obj, attr);
   }
-  template<class C>
-  static void on_has_many(const char * /*id*/, C &, const char * /*join_column*/, const utils::foreign_attributes & ) {}
-  template<class C>
-  static void on_has_many_to_many(const char * /*id*/, C &, const char * /*join_column*/, const char * /*inverse_join_column*/, const utils::foreign_attributes & ) {}
+  template<class Collection>
+  static void on_has_many(const char * /*id*/, Collection &con, const char * /*join_column*/, const utils::foreign_attributes & ) {}
+  template<class Collection>
+  void on_has_many_to_many(const char *id, Collection &con, const char *join_column, const char *inverse_join_column, const utils::foreign_attributes & ) {
+    if (id == nullptr || join_column == nullptr || inverse_join_column == nullptr) {
+      return;
+    }
+    if (current_entity_pk_ == 0ULL) {
+      // Without a local PK we cannot create relation rows
+      // (PK may be assigned later by Identity; that case needs a different mechanism than requested here)
+      return;
+    }
+
+    const auto rel_it = schema_.find(std::string{id});
+    if (rel_it == schema_.end()) {
+      throw query_builder_exception(query_build_error::UnknownType);
+    }
+    const table &rel_table = rel_it->second.table();
+
+    for (auto &obj : con) {
+      if (!obj) {
+        continue;
+      }
+
+      // Ensure target exists as dependency (deps first)
+      if (!obj.is_persistent()) {
+        using dep_t = std::remove_reference_t<decltype(*obj)>;
+        build_for<dep_t>(obj);
+      }
+
+      // Extract FK value from the foreign object
+      pk_value_extractor fk{};
+      access::process(fk, *obj);
+      if (fk.value == 0ULL) {
+        continue;
+      }
+
+      // Build INSERT into relation table with the 2 FK columns
+      insert_step rel_step{};
+      rel_step.query = executable_query{
+        query::query::insert()
+          .into(rel_table, {table_column{&rel_table, join_column}, table_column{&rel_table, inverse_join_column}})
+          .values({utils::database_type{current_entity_pk_}, utils::database_type{fk.value}})
+      };
+      relation_steps_.push_back(std::move(rel_step));
+    }
+  }
   template<class C>
   static void on_has_many_to_many(const char * /*id*/, C &, const utils::foreign_attributes & ) {}
 
@@ -204,7 +275,10 @@ private:
   const basic_schema &schema_;
 
   std::vector<insert_step> steps_;
+  std::vector<insert_step> relation_steps_;
   std::unordered_set<std::pair<std::type_index, const void *>, visit_key_hash> visited_;
+
+  std::uint64_t current_entity_pk_{0};
 };
 }
 #endif //MATADOR_INSERT_QUERY_BUILDER_HPP