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.

def _assert_correct_model(model_to_check, model_reference, obj_name):
        """
        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.
        """
        if not issubclass(model_to_check, model_reference):
            raise ConfigurationException('The %s model must be a subclass of %s'
                                         % (obj_name, model_reference.__name__))