code: def for_object(self, instance, flag=''):
          
          check(instance)
          content_type = ContentType.objects.get_for_model(instance).pk
          queryset = self.filter(content_type=content_type, object_id=instance.pk)
          if flag:
              queryset = queryset.filter(flag=flag)
          return queryset
docs: Filter to a specific instance.