publisher_categories_list_api_view.py

class PublisherCategoriesListAPIView(generics.ListAPIView):
    serializer_class = PublisherCategorySerializer
    queryset = Publisher.objects.all().values('id')

    def get_serializer_context(self):
        ctx = super().get_serializer_context()
        publishers = Publisher.objects.all().values('id', 'name', 'category_membership__category_id')
        publishers_dict = {}
        for publisher in publishers:
            if publishers_dict.get(publisher['id']) is None:
                publishers_dict[publisher['id']] = {
                    'publisher_id': publisher['id'],
                    'name': publisher['name'],
                    'categories': set(),
                }
            if publisher['category_membership__category_id']:
                publishers_dict[publisher['id']]['categories'].add(publisher['category_membership__category_id'])
        ctx['data'] = publishers_dict
        return ctx