query/todo.md

Todo

• move prepare_* methods from dialect to query_compiler

• add insert_query_builder and update_update_builder (returning multiple statements)

• finish fetch eager many-to-many relations

• implement lazy loading

• implement polymorphic class hierarchies

• finish database (schema_repository) class (move add/drop from session to schema)

• implement a flag class for enumerations

• PK generator

1. Introduce execute_result
2. Add abstract_pk_generator class
3. Add identity_pk_generator class
4. Add sequence_pk_generator class
5. Add table_pk_generator class
6. Add manual_pk_generator class
7. Extend session::insert logic to use pk generator
8. Add generator to schema_node or table class when attaching type

struct abstract_pk_generator {
  virtual ~abstract_pk_generator() = default;
  virtual int64_t next_id() = 0;
  virtual std::vector<int64_t> next_ids(int count) = 0;
};

struct sequence_pk_generator : abstract_pk_generator {
  explicit sequence_pk_generator(const std::string& sequence_name)
  : query_(query::query::select().nextval(sequence_name)) {}
  int64_t next_id(sql::executor& exec) override {
    return query_.fetch_value<int64_t>(exec);
  }
  std::vector<int64_t> next_ids(int count) override;
private:
  query::fetchable_query query_;
};

struct table_pk_generator : abstract_pk_generator {
  explicit table_pk_generator(const std::string& table_name);
  int64_t next_id() override;
  std::vector<int64_t> next_ids(int count) override;
private:
  std::string table_name_;
};

Proposal for polymorphic classes:

object_ptr::as does the following checks;

1. The requested type has a super class
2. Super class has a discriminator column defined
3. Super class has a discriminator value defined
4. Discriminator value is mapped to the requested type

If all checks succeed, the requested is fetched from the database.

schema.attach<jobs::Payload>("payloads", make_polymorph("type"));
schema.attach<jobs::Payload, jobs::IdPayload>("id_list_payloads", {"IdPayload"});
schema.attach<jobs::Payload, jobs::IdListPayload>("id_payloads", {"IdListPayload"});
//schema.attach<jobs::Payload, jobs::IdPayload>("id_list_payloads", make_polymorph_type("IdPayload"));
//schema.attach<jobs::Payload, jobs::IdListPayload>("id_payloads", make_polymorph_type("IdListPayload"));
object::object_ptr<jobs::Payload> payload;
auto result = payload.as<jobs::IdPayload>();
if (result.is_ok()) {
  const auto& is_payload = result.value();
  // Use requested type
  id_payload->id = 1;
}
payload.is_polymorphic();
payload.is_polymorphic_type<jobs::IdPayload>();