class ProductQuerySet(
        FlagMessageQuerySetMixin, ModuleItemFilterQuerysetMixin, BaseQuerySet):

    valid_module_filter_field = ('min_ranking', 'max_ranking')

    def sort_by_nullable_field(self, field):
        statement = 'CASE WHEN {db_table}.{field} IS NULL THEN 0 ELSE 1 END'\
            .format(db_table=Product,
                    field=field[1:] if field[0] == '-' else field)
        return self.extra(select={'null_position': statement})\
            .order_by('-null_position', field)

    def include_all_related(self):
        return self.select_related('brand').select_related('badge')\
            .prefetch_related('categories')

    def include_user_following(self, user):
        if not user or not user.is_authenticated:
            return self.extra({'user_following': False})
        else:
            select = 'SELECT followers.id ' \
                     'FROM {follow} AS followers ' \
                     'WHERE followers.user_id = {user_id} ' \
                     'AND followers.product_id = marketplace_product.id ' \
                     'LIMIT 1'.format(follow=ProductFollower._meta.db_table,
                                      user_id=user.id)
            return self.extra(select={'user_following': select})

    def include_user_review_id(self, user):
        if not user or not user.is_authenticated:
            return self.extra()
        else:
            select = 'SELECT r.entry_ptr_id FROM {review} AS r ' \
                     'JOIN {entry} as e ON e.id = r.entry_ptr_id ' \
                     'WHERE e.author_id = {user_id} ' \
                     'AND r.product_id = marketplace_product.id ' \
                     'LIMIT 1'.format(review=Review._meta.db_table,
                                      entry=Entry._meta.db_table,
                                      user_id=user.id)
            return self.extra(select={'user_review_id': select})