django annotate whether exists or not

Question

I have a query I'm using:

people = Person.objects.all().annotate(num_pets=Count('pets'))
for p in people:
    print(p.name, p.num_pets == 0)

(Pet is ManyToOne with Person)

But i'm actually not interested in the number of pets, but only on whether a person has any pets or not. How can this be done?

Answer 1

You can make use of an Exists expression ^[Django-doc] to determine if there exists a Pet for that Person. For example:

from django.db.models import Exists, OuterRef

Person.objects.annotate(
    has_pet=Exists(Pet.objects.filter(person=OuterRef('pk')))
)

Here the model is thus Pet that has a ForeignKey named person to Person. If the fields are named differently, then you should of course update the query accordingly.