django model 如何实现过滤重复计数

使用 raw 查询，提示没有主键，InvalidQuery('Raw query must include the primary key')，openid 非主键

answer_num = cls.objects.raw("SELECT COUNT(*) FROM server_answerlog WHERE `paper_id` = %d GROUP BY `openid`" % int(paper_id))

使用

answer_num = cls.objects.filter(paper_id=paper_id).values('openid').annotate(num=Count('openid'))

生成出来了并没有我想要的 group by openid

SELECT `server_answerlog`.`openid`, COUNT(`server_answerlog`.`openid`) AS `num` FROM `server_answerlog` WHERE `server_answerlog`.`paper_id` = 1 GROUP BY `server_answerlog`.`id` ORDER BY `server_answerlog`.`id` ASC

对 annotate 不太熟悉，特来求助各位大佬