Annotate over a field of intermediate model django - django

Question

I have the following scheme:
class User(AbstractUser):
pass
class Task(models.Model):
pass
class Contest(models.Model):
tasks = models.ManyToManyField('Task',
related_name='contests',
blank=True,
through='ContestTaskRelationship')
participants = models.ManyToManyField('User',
related_name='contests_participated',
blank=True,
through='ContestParticipantRelationship')
class ContestTaskRelationship(models.Model):
contest = models.ForeignKey('Contest', on_delete=models.CASCADE)
task = models.ForeignKey('Task', on_delete=models.CASCADE)
cost = models.IntegerField()
class ContestParticipantRelationship(models.Model):
contest = models.ForeignKey('Contest', on_delete=models.CASCADE)
user = models.ForeignKey('User', on_delete=models.CASCADE)
task = models.ForeignKey('Task', on_delete=models.CASCADE, related_name='contests_participants_relationship')
is_solved = models.BooleanField()
Now I get the contest object and need to fetch all tasks over the tasks field, rach annotated with count of users solved it. So, I need to count the number of ContestParticipantRelationship with needed task, needed contest and is_solved set to True. How to make such a query?

Answer 1

Probably something like:
from django.db.models import IntegerField, Value, Sum
from django.db.models.functions import Cast, Coalesce
Task.objects.filter(
contests__contest=some_contest,
).annotate(
nsolved=Cast(Coalesce(
Sum('contests_participants_relationship__is_solved'), Value(0)
),IntegerField())
)
So here we first filter on the fact that the contest of the task is some_contest. And next we perform a Sum(..) over the is_solved column. Since there are corner-cases where this can be NULL (in case there is no user that made an attempt, etc.), then we convert it to a 0, and furthermore we cast it to an IntegerField, since otherwise some instances might be annotated with True, and False in case zero or one user solves it.