Can't use unique_together if one of the fields is in a mixin

[Tom Carrick]

Slightly related to https://code.djangoproject.com/ticket/16732

I have a mixin:

class PublishableMixin(object):
    publish_date = models.DateTimeField()

and a model I wish to use it in:

class Article(PublishableMixin, models.Model):
    slug = models.SlugField()

    class Meta:
        unique_together = ('publish_date', 'slug')

This fails at the system check level:

news.Article: (models.E012) 'unique_together' refers to the non-existent field 'publish_date'.

[Tim Graham]

You need to use abstract models as described ​in the documentation (not inherit from object). If something isn't clear there about that, could you open a documentation ticket? Thanks!