code: def for_model(self, model):
          
          ct = ContentType.objects.get_for_model(model)
          qs = self.get_queryset().filter(content_type=ct)
          if isinstance(model, models.Model):
              qs = qs.filter(object_pk=force_text(model._get_pk_val()))
          return qs
docs: QuerySet for all comments for a particular model (either an instance or
          a class).