'QuerySet for all comments for a particular model (either an instance or
a class).'

def for_model(self, model):

    ct = ContentType.objects.get_for_model(model)
    qs = self.get_query_set().filter(content_type=ct)
    if isinstance(model, models.Model):
        qs = qs.filter(object_pk=force_text(model._get_pk_val()))
    return qs
