order_by("field1__-field2") yields inconsistent behavior

[Thomas C]

In our codebase, someone used SomeModel.objects.order_by("foo__-created_at") to order according to a foreign key's field and it was working (yielding the correct query with DESC order).

However, after we added the following index on SomeModel:

    class Meta:
        indexes = [
            models.Index(fields=["some_other_field", "created_at"]),
        ]

The same code SomeModel.objects.order_by("foo__-created_at") stopped working and started raising an error:

django.core.exceptions.FieldError: Cannot resolve keyword '-created_at' into field. Choices are: context, created_at, id, tag_readings, updated_at, user_id, warehouse_id

Shouldn't the pattern for ordering in a descending order with chaining be documented?

According to ​https://github.com/django/django/blob/main/django/db/models/sql/compiler.py#L402-L403 it should be -foo__created_at.
Also, we should expect the wrong pattern to fail unconditionally.