In a Django QuerySet, how to filter for “not exists” in a many-to-one relationship

Question

I have two models like this:

class User(models.Model):
    email = models.EmailField()

class Report(models.Model):
    user = models.ForeignKey(User)
         


        

Answer 1

New in Django 1.11 you can add EXISTS subqueries:

User.objects.annotate(
    no_reports=~Exists(Reports.objects.filter(user__eq=OuterRef('pk')))
).filter(
    email__startswith='a',
    no_reports=True
)

This generates SQL something like this:

SELECT
    user.pk,
    user.email,
    NOT EXISTS (SELECT U0.pk FROM reports U0 WHERE U0.user = user.pk) AS no_reports
FROM user
WHERE email LIKE 'a%' AND NOT EXISTS (SELECT U0.pk FROM reports U0 WHERE U0.user = user.pk);

A NOT EXISTS clause is almost always the most efficient way to do a "not exists" filter.

---

As of Django 3.0 you can now use expressions directly in a filter(), removing the unnecessary SQL clause:

User.objects.filter(
    email__startswith='a',
    ~Exists(Reports.objects.filter(user__eq=OuterRef('pk')))
)

SELECT user.pk, user.email
FROM user
WHERE email LIKE 'a%' AND NOT EXISTS (SELECT U0.pk FROM reports U0 WHERE U0.user = user.pk);