Question

I have model A with a fk to model B

Now I need to make a queryset that gets all B’s with less then 5 A’s linked to them.

Edit Maybe I should have mentioned this is a Django project and what I’m looking for is a Queryset reference thats efficient enough to be run quite a number of times

i’ve tried something like:

B.objects.select_related()

but I don’t understand how to limit this qs to only include the B’s with max 5 occurrences

Answer 1

Use annotations:

from django.db.models import Count

B.objects.annotate(a_count=Count('a')).filter(a_count__lt=5)

See: https://docs.djangoproject.com/en/dev/topics/db/aggregation/