You can’t.
The unique_together
clause is directly translated to the SQL
unique index. And you can only set those on columns of a single table, not a combination of several tables.
You can add validation for it yourself though, simply overwrite the validate_unique
method and add this validation to it.
Docs: http://docs.djangoproject.com/en/dev/ref/models/instances/#django.db.models.Model.validate_unique