Django: GenericForeignKey and unique_together

Question

In the application I'm working on I'm trying to share access tokens within a company. Example: a local office can use the headquarter's tokens to post something on their Facebook page.

class AccessToken(models.Model):
    """Abstract class for Access tokens."""
    owner = models.ForeignKey('publish.Publisher')
    socialMediaChannel = models.IntegerField(
        choices=socialMediaChannelList, null=False, blank=False
    )
    lastUpdate = models.DateField(auto_now=True)

    class Meta:
        abstract = True

Since Facebook, Twitter and other social media sites handle access tokens in their own way I made and abstract class AccessToken. Each site gets its own class e.g.

class FacebookAccessToken(AccessToken):
    # class stuff

After doing some reading I found out that I must use a GenericForeignKey to point to classes that inherit AccessToken. I made the following class:

class ShareAccessToken(models.Model):
    """Share access tokens with other publishers."""
    sharedWith = models.ForeignKey('publish.Publisher')
    sharedBy = models.ForeignKey(User)

    # for foreignkey to abstract model's children
    contentType = models.ForeignKey(ContentType)
    objectId = models.PositiveIntegerField()
    contentObject = GenericForeignKey('contentType', 'objectId')

    class Meta:
        unique_together = (('contentObject', 'sharedWith'))

When I run the django test server I get the following error:

core.ShareAccessToken: (models.E016) 'unique_together' refers to field 'contentObject' which is not local to model 'ShareAccessToken'. HINT: This issue may be caused by multi-table inheritance.

I don't understand why I get this error, first time using GenericForeignKey. What am I doing wrong?

If there is a smarter way to share the access tokens I would love to hear about it.

Answer 1

Your use of the generic foreign key in this situation is correct.

The error is coming from your unique_together declaration in your model. unique_together can only be used with columns that exist in the database. Since contentObject is not a real column, Django complains about the constraint.

Instead, you can do the following:

unique_together = (('contentType', 'contentId', 'sharedWidth'),)

This is equivalent to what you had defined in your question because contentObject is really just the combination of contentType and contentId behind the scenes.