CursorPagination doesn't process ordering from django-filters

[ulgens]

After migrating one of our projects to cursor pagination, we lost the ordering functionality in all related API endpoints. After a deep dive, I found that the related code block on CursorPagination class

        # Cursor pagination always enforces an ordering.
        if reverse:
            queryset = queryset.order_by(*_reverse_ordering(self.ordering))
        else:
            queryset = queryset.order_by(*self.ordering)

https://github.com/encode/django-rest-framework/blob/main/rest_framework/pagination.py#L626

connected to the .get_ordering() of the same class that is not compatible with django_filters.filters.OrderingFilter. That class applies the filtering via its own .filter() class and doesn't expose the selected ordering options to the outside, like rest_framework.filters.OrderingFilter.get_ordering. That missing interface leads to CursorPagination.paginate_queryset() overriding any ordering provided by django-filters.

I tried to understand the goal here, and it seems the code is trying to guarantee that an ordering is present, but it does so without checking first.

GenericAPIView.filter_queryset() is already applying all the filters available, and thanks to that, I believe CursorPagination can have a little less customization and working integration with django-filters by having two changes:

• .get_ordering() is called only if queryset.ordered is false. If the goal is to enforce ordering, no need to apply an additional ordering to a queryset that is already ordered.
• Because all filters are already applied by GenericAPIView.filter_queryset(), and the enforced ordering is applied only when no other ordering is done (thanks to the previous step), CursorPagination.get_ordering() won't need to process ordering classes.

An additional benefit of this approach is eliminating the issue defined in #9371 by completely removing the related code block.

The resulting set of changes looks like this: https://github.com/ulgens/django-rest-framework/pull/1/files