Filtering queryset to get empty related set

Question

I'd like to get a QuerySet (to feed a ModelChoiceField) listing all objects that have an empty ForeignKey on the related set.

An example extract of my models:

class Bankcard(models.Model):
    name = models.CharField(max_length=128)
    number = models.IntegerField()
    ...

class Person(models.Model):
    bankcard = models.ForeignKey(Bankcard, blank=True, null=True)
    first_name = models.CharField(max_length=64)
    last_name = models.CharField(max_length=64)
    ...

In a form I'd like to list all bank cards that have not been linked to a person yet.

What I tried is to do my own filtering as part of a list:

bankcards = Bankcard.objects.all()
unbound_cards = []
for card in bankcards:
    if len(card.person_set.all()) == 0:
        unbound_cards.append(card)

... but that doesn't work as I need to provide a QuerySet to the ModelChoiceField and not a list and I get 'list' object has no attribute 'all'.

So I tried filtering directly on the related set:

unbound_cards = Bankcard.objects.filter(person_set__isnull = True)

That doesn't work either: django.core.exceptions.FieldError: Cannot resolve keyword 'person_set' into field. Choices are: ...

Any idea?

Answer 1

Try:

Bankcard.objects.filter(person__isnull=True)