Django 6.1: TypeError in Query.add_select_col() when combining QuerySets with Meta.ordering referencing related model fields

[Ulrich Görtz]

The following problem occurs with Django 6.1, PostgreSQL, Python 3.14.

For a model (with a regular primary key) and Meta.ordering referencing fields through a foreign key (ordering = ('user__lastname', ), say), QuerySet.difference() applied to two QuerySets of that model (e.g., as a trivial example, A.objects.all().difference(A.objects.all())) throws a TypeError during SQL compilation because query.selected is None.

Adding .order_by() without arguments to the difference method resolves the problem.

The same query worked in Django 6.0. The problem might be related to #37466

Thanks for your work.