Django unique_together on model parent class fields

Question

I want to generalize my workflow with models where presented GenericForeignKey fields.

So I create parent class GFKModel:

class GFKModel(models.Model):
    target_content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    target_id = models.PositiveIntegerField()
    target = GenericForeignKey('target_content_type', 'target_id')

Then I inherit it:

class Question(GFKModel):
    author = models.ForeignKey(User)
    text = models.TextField()

    class Meta:
        unique_together = ('author', 'target_content_type', 'target_id')

I need to add unique_together constraint on both 'author', 'target_content_type' and 'target_id', but I can't do that due to migration error:

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

How can I do that?

Answer 1

I've missed declaration of GFKModel as 'abstract' class:

class GFKModel(models.Model):
    target_content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    target_id = models.PositiveIntegerField()
    target = GenericForeignKey('target_content_type', 'target_id')

    class Meta:
        abstract = True

Now it works as expected.