我有两个带有复合键的模型：
class ContestUser(models.Model):
user_id = models.IntegerField(primary_key = True)
contest_id = models.IntegerField(primary_key = True)
username = models.CharField(max_length = 1536, blank = True)
.
.
.
class ContestRegistration(models.Model):
user_id = models.IntegerField(primary_key = True)
contest_id = models.IntegerField(primary_key = True)
status = models.IntegerField(choices = EJUDGE_CONTEST_STATUSES)
.
.
.
第一个问题是我如何将它们关联起来，并像在连接中一样查询。
Select * from ContestRegistration r join ContestUser u on r.user_id = u.user_id and r.contest_id = u.contest_id where r.contest_id = 3;
其次是如何保存这样的对象？
cuser = ContestUser.objects.get(user_id = 1, contest_id = 1)
cuser.username = 'username'
cuser.save()
这会导致 IntegrityError: (1062, "Duplicate entry '1-1' for key 'PRIMARY'")
执行的SQL是：
SELECT * FROM `users` WHERE (`users`.`contest_id` = 1 AND `users`.`user_id` = 1 );
SELECT (1) AS `a` FROM `users` WHERE `users`.`user_id` = 1 LIMIT 1;
UPDATE `users` SET ... WHERE `users`.`user_id` = 1 ;