implemented lazy loading for object_ptr and belongs_to

test/models/shipment.hpp

@@ -13,6 +13,10 @@ namespace matador::test {
 struct package;
 
 struct shipment {
+  shipment() = default;
+  shipment(const long id, std::string tracking_number)
+  : id(id), tracking_number(std::move(tracking_number)) {}
+
   long id{};
   std::string tracking_number;
   object::collection<object::object_ptr<package> > packages{};
@@ -27,6 +31,10 @@ struct shipment {
 };
 
 struct package {
+  package() = default;
+  package(const long id, double weight, object::object_ptr<shipment> del)
+  : id(id), weight(weight), delivery(std::move(del)){}
+
   long id{};
   double weight{};
   object::object_ptr<shipment> delivery;