ModelDiffMixin fix

@property
def _dict(self):
    from django.db.models.query_utils import DeferredAttribute
    exclude = [f.name for f in self._meta.fields if
               isinstance(self.__class__.__dict__.get(f.attname),
                          DeferredAttribute)]

    return model_to_dict(self, fields=[field.name for field in
                         self._meta.fields], exclude=exclude)