Django subquery with aggregate

Question

I have two models called User and Transaction . Here i want to get the all the users with total sum of the transaction amount where status is success.

I have tried with subquery but i am not getting how to annotate the aggregate of the subquery with conditions

class User(models.Model):
  name = models.CharField(max_length=128)

class Transaction(models.Model):
  user = models.ForeignKey(User)
  status = models.CharField(choices=(("success", "Success"),("failed", "Failed")))
   amount = models.DecimalField(max_digits=10, decimal_places=2)

subquery = Transaction.objects.filter(status="success", user=OuterRef('pk')).aggregate(total_spent = Coalesce(Sum('amount'), 0))

query = User.objects.annotate(total_spent=Subquery(subquery:how to do here ?)).order_by(how to order here by total_spent)

Answer 1

This is made a lot easier with the django-sql-utils package.

from django.db.models import Sum,
from sql_util.utils import SubqueryAggregate

User.objects.annotate(
    total_spend=SubqueryAggregate('transaction__amount',
                                  filter=Q(status='success'),
                                  aggregate=Sum)
)

If you want to do it the long way (without django-sql-utils), you need to know these two things about the subquery:

1. It can't be evaluated before it is used

2. It can only return a single record with a single column

So, you can't call aggregate on the subquery, because this evaluates the subquery immediately. Instead you have to annotate the value. You also have to group by the outer ref value, otherwise you'll just annotate each Transaction independently.

subquery = Transaction.objects.filter(
        status='success', user=OuterRef('pk')
    ).values(
        'user__pk'
    ).annotate(
        total_spend=Sum('amount')
    ).values(
        'total_spend'
    )

The first .values causes the correct group by. The second .values causes selecting the one value that you want.