Creating join paths to fields that are GenericRelations to proxy models fails with an AttributeError

[Rachel Tobin]

This appears to be a regression introduced by https://code.djangoproject.com/ticket/12885. When attempting to build joins for a GenericRelation field that points to a proxy model, an AttributeError is caused via contrib/contenttypes/fields.py:375 , when trying to get the ancestor of the proxy model's parent. Since the proxy model's parent does not have an ancestor, None is returned and contrib/contenttypes/fields.py:377 raises an AttributeError, given that field is None.

As an example:

class GenericConcreteModel(models.Model):
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey()


class GenericProxyModel(GenericConcreteModel):
    class Meta:
        proxy = True


class GenericRelationProxyModel(models.Model):
    proxy_model = GenericRelation('GenericProxyModel')

Where the query

GenericRelationProxyModel.objects.filter(proxy_model__content_type=ContentType.objects.get(id=1))

will cause an attribute error.