Question

Sounds so simple, but has me stymied:

class A(models.Model):
    various fields

class P(models.Model):
    various fields
    a = models.ForeignKey(A)

object_a = A.objects.get(pk=1)
p_objects = P.objects.filter(a=objecta)    # one way
p_objects = A.p_set.all()                  # other way

to get all the P’s that belong object_a is easy. What I can’t figure out is how to get all the A objects that have no associated P object.

I can’t reverse the ForeignKey relation because each A might have more than one P.

Answer 1

This will work

A.objects.filter(p__isnull=True)