Django - UserProfile m2m field in admin - error

Question

My models:

class UserProfile(models.Model):
    TYPES_CHOICES = (
        (0, _(u'teacher')),
        (1, _(u'student')),
    )
    user = models.ForeignKey(User, unique=True)
    type = models.SmallIntegerField(default=0, choices=TYPES_CHOICES, db_index=True)
    cities = models.ManyToManyField(City)
class City(models.Model):
    name = models.CharField(max_length=50)
    slug = models.SlugField(max_length=50)

In admin.py:

admin.site.unregister(User) 
class UserProfileInline(admin.StackedInline):
    model = UserProfile

class UserProfileAdmin(UserAdmin):
    inlines = [UserProfileInline]

admin.site.register(User, UserProfileAdmin)

@receiver(post_save, sender=User)
def create_profile(sender, instance, created, **kwargs):
    """Create a matching profile whenever a user object is created."""
    if created:
        profile, new = UserProfile.objects.get_or_create(user=instance)

But when I add new user and select a city I get that error: IntegrityError at /admin/auth/user/add/ (1062, "Duplicate entry '3' for key 'user_id'")

What is wrong with my code? If I don't select any city - user is added properly. Some way, user is being added to UserProfile more than once.

Answer 1

I had this same issue recently. It actually makes perfect sense when you think about it. When you save a form with inlines in the admin, it saves the main model first, and then proceeds to save each inline. When it saves the model, your post_save signal is fired off and a UserProfile is created to match, but now it's time to save the inlines. The UserProfile inline is considered new, because it didn't exist previously (has no pk value), so it tries to save as an entirely new and different UserProfile and you get that integrity error for violating the unique constraint. The solution is simple. Just override UserProfile.save:

def save(self, *args, **kwargs):
    if not self.pk:
        try:
            p = UserProfile.objects.get(user=self.user)
            self.pk = p.pk
        except UserProfile.DoesNotExist:
            pass

    super(UserProfile, self).save(*args, **kwargs)

Essentially, this just checks if there's an existing UserProfile for the user in question. If so, it sets this UserProfile's pk to that one's so that Django does an update instead of a create.