implemented lazy loading for object_ptr and belongs_to

source/orm/sql/field.cpp

@@ -35,6 +35,17 @@ field &field::operator=(field &&x) noexcept {
   return *this;
 }
 
+bool field::operator==(const field &rhs) const {
+  return name_ == rhs.name_ &&
+    type_ == rhs.type_ &&
+    index_ == rhs.index_ &&
+    value_ == rhs.value_;
+}
+
+bool field::operator!=(const field &rhs) const {
+  return !operator==(rhs);
+}
+
 const std::string &field::name() const {
   return name_;
 }