Relation between models with composite keys - django

Question

I have two models with composite keys:
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)
.
.
.
First question is How can I relate them, and query like in join.
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;
Second is How to save an object like this?
cuser = ContestUser.objects.get(user_id = 1, contest_id = 1)
cuser.username = 'username'
cuser.save()
This results in IntegrityError: (1062, "Duplicate entry '1-1' for key 'PRIMARY'")
Executed SQL is:
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 ;

Answer 1

Django models don't support multiple primary keys: https://docs.djangoproject.com/en/1.3/faq/models/#do-django-models-support-multiple-column-primary-keys
However, as the documentation describes, you can use other properties on ForeignKey fields, like unique_together to do the same thing. Hope that helps you out.