subquery

subquery.py

from django.db.models import OuterRef, Subquery

unique_users_query = (CouponApplication
    .objects
    .all()
    .filter(coupon=OuterRef('id'))
    .values('user_id') # group cities by state
    .order_by() # reset ordering
    .annotate(cnt=Count('*'))
    .values('cnt')[:1] # Only have 1 row & 1 value allowed
)

Coupon.objects.all()
    .annotate(
        unique_users_count=Subquery(
            city_query, 
            output_field=models.IntegerField()
        )
)