code: def _assert_correct_model(model_to_check, model_reference, obj_name):
          
          if not issubclass(model_to_check, model_reference):
              raise ConfigurationException('The %s model must be a subclass of %s'
                                           % (obj_name, model_reference.__name__))
docs: Helper that asserts the model_to_check is the model_reference or one of
          its subclasses. If not, raise an ImplementationError, using "obj_name"
          to describe the name of the argument.