How to create new field in queryset instead of usign annotation?

Next code not working right

    def get_queryset(self, request):

        qs = super(QuestionAdmin, self).get_queryset(request)

        qs = qs.annotate(

            count_answers=models.Count('answers', distinct=True),

            count_tags=models.Count('tags', distinct=True),

            count_opinions=models.Count('opinions', distinct=True),

            scope=models.Sum(

                models.Case(

                    models.When(opinions__is_useful=True, then=1),

                    models.When(opinions__is_useful=False, then=-1),

                    output_field=models.IntegerField()

                ), distinct=True

            ),

        )

        return qs

I am tried made new field manually as:

    def get_queryset(self, request):

        qs = super(QuestionAdmin, self).get_queryset(request)

        qs = qs.annotate(

            count_answers=models.Count('answers', distinct=True),

            count_tags=models.Count('tags', distinct=True),

            count_opinions=models.Count('opinions', distinct=True),

        )

        (setattr(i, 'scope', i.get_scope()) for i in qs)

        return qs

But Django not visible my new field (error from traceback):

Cannot resolve keyword 'scope' into field. Choices are: answers, author, author_id, count_answers, count_opinions, count_tags, date_added, date_modified, id, is_dublicated, opinions, slug, status, tags, text_question, title, views

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