docs: def _load_fields(new_class, attrs):
          """Load field items into Class.
  
          This method sets up the primary attribute of an association.
          If Child class has defined an attribute so `parent = field.Reference(Parent)`, then `parent`
          is set up in this method, while `parent_id` is set up in `_set_up_reference_fields()`.
          """
          for attr_name, attr_obj in attrs.items():
              if isinstance(attr_obj, (Field, Reference)):
                  setattr(new_class, attr_name, attr_obj)
                  new_class.meta_.declared_fields[attr_name] = attr_obj