implemented lazy loading for object_ptr and belongs_to

test/models/book.hpp

@@ -12,6 +12,9 @@ namespace matador::test {
 struct author;
 
 struct book {
+  book() = default;
+  book(const unsigned int id, std::string title, object::object_ptr<author> author, unsigned short published_in)
+  : id(id), title(std::move(title)), book_author(std::move(author)), published_in(published_in) {}
   unsigned int id{};
   std::string title;
   object::object_ptr<author> book_author;