docs: def for_model(self, model, content_type=None):
          ''' Returns all objects that are attached to given model '''
          content_type = content_type or ContentType.objects.get_for_model(model)
          kwargs = {
                      self.ct_field: content_type,
                      self.fk_field: model.pk
                   }
          objects = self.get_query_set().filter(**kwargs)
          return objects