django conditional annotate

django_annotate_queryset.py

class OrderQuerySet(models.QuerySet):
    def annotate_sold_coupon(self):
        sold_coupons = SoldCoupon.objects.filter(sell_code=OuterRef('sold_coupon__sell_code'))
        return self.annotate(
            sold_coupon__sell_code=Case(
                When(site_id=2, then=Concat(Value('SNAPP-NEW-'), 'id')),
                When(site_id=3, then=Concat(Value('TAP30-NEW-'), 'id')),
                When(site_id=4, then=Concat(Value('T2BON-NEW-'), 'id'))
            ),
            sold_coupon__exist=Exists(sold_coupons.values('is_used')),
            sold_coupon__id=Subquery(sold_coupons.values('id')),
            sold_coupon__is_used=Subquery(sold_coupons.values('is_used'))
        )