Django: Why do some model fields clash with each other?

I want to create an object that contains 2 links to Users. For example:

class GameClaim(models.Model):
    target = models.ForeignKey(User)
    claimer = models.ForeignKey(User)
    isAccepted = models.BooleanField()

but I am getting the following errors when running the server:

• Accessor for field 'target' clashes with related field 'User.gameclaim_set'. Add a related_name argument to the definition for 'target'.

• Accessor for field 'claimer' clashes with related field 'User.gameclaim_set'. Add a related_name argument to the definition for 'claimer'.

Can you please explain why I am getting the errors and how to fix them?

Solution 1:

You have two foreign keys to User. Django automatically creates a reverse relation from User back to GameClaim, which is usually gameclaim_set. However, because you have two FKs, you would have two gameclaim_set attributes, which is obviously impossible. So you need to tell Django what name to use for the reverse relation.

Use the related_name attribute in the FK definition. e.g.

class GameClaim(models.Model):
    target = models.ForeignKey(User, related_name='gameclaim_targets')
    claimer = models.ForeignKey(User, related_name='gameclaim_users')
    isAccepted = models.BooleanField()

Solution 2:

The User model is trying to create two fields with the same name, one for the GameClaims that have that User as the target, and another for the GameClaims that have that User as the claimer. Here's the docs on related_name, which is Django's way of letting you set the names of the attributes so the autogenerated ones don't conflict.

Solution 3:

The OP isn't using a abstract base class... but if you are, you will find that hard coding the related_name in the FK (e.g. ..., related_name="myname") will result in a number of these conflict errors - one for each inherited class from the base class. The link provided below contains the workaround, which is simple, but definitely not obvious.

From the django docs...

If you are using the related_name attribute on a ForeignKey or ManyToManyField, you must always specify a unique reverse name for the field. This would normally cause a problem in abstract base classes, since the fields on this class are included into each of the child classes, with exactly the same values for the attributes (including related_name) each time.

More info here.