Retain Queryset ordering when using cursor based pagination

[KushGoyal]

Cursor pagination enforces ordering of queryset since that is a mandatory requirement for it to work. The ordering applied should be unique.

ListModelMixin calls pagination after applying all the filter backends which includes ordering filter. When cursor pagination class gets the queryset it applies the ordering filter again or uses the ordering attribute set on the class. This causes the last ordering to be applied on the queryset which is the one applied by the cursor pagination class.

My suggestion is to allow the user to retain the ordering already applied at various points in the code so that cursor pagination does not override it.

2 changes are suggested:

1. Add field retain_ordering to cursor pagination class which controls if the ordering should be retained or not.
2. Override the get_ordering method to check this field and retain the ordering if set to True

    def get_ordering(self, request, queryset, view):
        if self.retain_ordering and queryset.ordered:
            ordering = queryset.query.order_by
        else:
            ordering = super().get_ordering(request, queryset, view)
        return ordering

I know that queryset.order_by is not documented but there is no other cleaner and simple way to get the applied ordering on a queryset.

I have called the super method since this code is from my custom cursor pagination class but of course in the else condition original get_ordering code will be called.