added sequence_pk_generator implementation

todo.md

@@ -17,7 +17,33 @@
 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
-9. add `make_object`function
+
+```cpp
+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:__
 
@@ -32,8 +58,10 @@ If all checks succeed, the requested is fetched from the database.
 
 ```cpp
 schema.attach<jobs::Payload>("payloads", make_polymorph("type"));
-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"));
+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()) {