Get cumsum from aggregated field with Django orm - django

Question

In my project, I want to get the sum of an "amount" field which comes form an aggregate. I've read some posts on this but I can't find a way to achieve what I want.
Example model:
class ScheduledOperation:
day = models.dateField()
amount = models.DecimalField(...)
Example queryset
{'day': datetime.date(2023, 2, 7), 'amount': Decimal('-500.00')} # same day each month
{'day': datetime.date(2023, 2, 7), 'amount': Decimal('1500.00')} # same day each month
{'day': datetime.date(2023, 3, 7), 'amount': Decimal('-500.00')}
{'day': datetime.date(2023, 3, 7), 'amount': Decimal('1500.00')}
{'day': datetime.date(2023, 4, 7), 'amount': Decimal('-500.00')}
{'day': datetime.date(2023, 4, 7), 'amount': Decimal('1500.00')}
{'day': datetime.date(2023, 5, 7), 'amount': Decimal('-500.00')}
{'day': datetime.date(2023, 5, 7), 'amount': Decimal('1500.00')}
{'day': datetime.date(2023, 5, 8), 'amount': Decimal('-4000.00')} # big op here
Where I am so far
ScheduledOperation.objects.order_by('day').values('day').annotate(day_tot=Sum('amount')) gives me the total amount for each day:
{'day': datetime.date(2023, 2, 7), 'day_tot': Decimal('1000')}
{'day': datetime.date(2023, 3, 7), 'day_tot': Decimal('1000')}
{'day': datetime.date(2023, 4, 7), 'day_tot': Decimal('1000')}
{'day': datetime.date(2023, 5, 7), 'day_tot': Decimal('1000')}
{'day': datetime.date(2023, 5, 8), 'day_tot': Decimal('-4000')}
What I want
{'day': datetime.date(2023, 2, 7), 'day_tot': Decimal('1000'), 'cumul_amount':Decimal('1000')}
{'day': datetime.date(2023, 3, 7), 'day_tot': Decimal('1000'), 'cumul_amount':Decimal('2000')}
{'day': datetime.date(2023, 4, 7), 'day_tot': Decimal('1000'), 'cumul_amount':Decimal('3000')}
{'day': datetime.date(2023, 5, 7), 'day_tot': Decimal('1000'), 'cumul_amount':Decimal('4000')}
{'day': datetime.date(2023, 5, 8), 'day_tot': Decimal('-4000'), 'cumul_amount':Decimal('0')}
What I tried
After reading other related posts on this subject, I've tried to use the Window function:
self.coming_scheduled_ops.order_by('day').values('day').annotate(
day_tot=Sum('amount')
).annotate(
cumul_amount=Window(
Sum('amount'),order_by='day'
)
)
but this does not work:
{'day': datetime.date(2023, 2, 7), 'day_tot': Decimal('1000'), 'cumul_amount': Decimal('1500')}
{'day': datetime.date(2023, 3, 7), 'day_tot': Decimal('1000'), 'cumul_amount': Decimal('3000')}
{'day': datetime.date(2023, 4, 7), 'day_tot': Decimal('1000'), 'cumul_amount': Decimal('4500')}
{'day': datetime.date(2023, 5, 7), 'day_tot': Decimal('1000'), 'cumul_amount': Decimal('6000')}
{'day': datetime.date(2023, 5, 8), 'day_tot': Decimal('-4000'), 'cumul_amount': Decimal('2000')}
I can't use Window(Sum('day_tot')), it throws django.core.exceptions.FieldError: Cannot compute Sum('day_tot'): 'day_tot' is an aggregate
Could someone help me understand the Window function plz?

Answer 1

I'm not sure if that's possible using Django ORM, but you can easily do this using Python.
queryset = ScheduledOperation.objects.all()
queryset = (
queryset.order_by('day')
.values('day')
.annotate(day_tot=Sum('amount'))
)
cumul_amount = 0
for day in queryset:
cumul_amount += day['day_tot']
day['cumul_amount'] = cumul_amount
It results in:
{'day': datetime.date(2023, 2, 7), 'day_tot': Decimal('1000'), 'cumul_amount': Decimal('1000')}
{'day': datetime.date(2023, 3, 7), 'day_tot': Decimal('1000'), 'cumul_amount': Decimal('2000')}
{'day': datetime.date(2023, 4, 7), 'day_tot': Decimal('1000'), 'cumul_amount': Decimal('3000')}
{'day': datetime.date(2023, 5, 7), 'day_tot': Decimal('1000'), 'cumul_amount': Decimal('4000')}
{'day': datetime.date(2023, 5, 8), 'day_tot': Decimal('-4000'), 'cumul_amount': Decimal('0')}