Django ORM annotate on foreign key query - django

Question

I have 3 models:
class Project(models.Model):
name = models.CharField(max_length=300, unique=True)
description = models.CharField(
max_length=2000,
blank=True,
null=True,
default=None
)
class QuestionSession(models.Model):
name = models.CharField(
max_length=500, default=None, null=True, blank=True
)
project = models.ForeignKey(
Project,
on_delete=models.CASCADE,
related_name='sessions',
blank=True,
null=True,
default=None
)
class Question(models.Model):
description = models.CharField(max_length=500)
question_session = models.ForeignKey(
QuestionSession,
on_delete=models.CASCADE,
related_name='questions',
blank=True,
null=True,
default=None
)
As you can see, Project contains Sessions, Session contains questions.
What I'm trying to achieve is I wanna fetch a single Project with sessions and number of questions in them. I can do it easily with 2 different queries but I cannot do it in 1 query.
My serializers:
class SingleProjectSerializer(serializers.ModelSerializer):
sessions = MinifiedSessionSerializer(many=True)
class Meta:
model = Project
fields = [
'id',
'name',
'description',
'sessions'
]
class MinifiedSessionSerializer(serializers.ModelSerializer):
questions_number = serializers.IntegerField()
class Meta:
model = QuestionSession
fields = [
'id',
'name',
'questions_number'
]
I used to grab sessions in a single query like this:
Project.objects.get(id=project_id).sessions.annotate(questions_number=Count('questions'))
But how to do it now? I need to fetch the project first and then annotate on sessions. I have no idea how to do it. I need a query like this:
Project.objects.filter(pk=project_id).annotate(sessions__questions_number=Count('sessions__questions'))

Answer 1

I don't believe it's possible through Django ORM. The only solution which I can think of is changing the way on how you ask for the data:
sessions = QuestionSession.objects.filter(project_id=project_id).select_related('project').annotate(questions_number=Count('questions'))
project = sessions[0].project
That would end up in a single query, but I assume you want to pass this project instance down to the DRF serializer. In such case the project knows nothing about prefetched sessions, so it would need to be handled separately. Additionally there is an issue when particular project has no associated sessions - sessions[0].project would raise exception. To keep the code clean I would probably stay with yours current approach (but then - the issue remains unresolved to keep everything in one db hit).