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

The best option for finding rows where there is a joining row:

Report.objects.filter(user__isnull=False).distinct()

This uses an INNER JOIN (and then redundantly checks User.id is not null).

The best option for finding rows where there is not a joining row:

Report.objects.filter(user__isnull=True)

This makes LEFT OUTER JOIN, then checks User.id is not null.

Queries based on joins will be quicker than subqueries, so this is quicker than newly available options such as in Django >= 3, for finding rows without a joining row:

Report.objects.filter(~Exists(User.objects.filter(report=OuterRef('pk'))))

This creates a WHERE NOT EXISTS (SELECT .. FROM User..) so involves a potentially large intermediate result set (thanks @Tomasz Gandor).

This for Django <3, where filter() can't be passed subqueries, also uses a subquery so is slower:

Report.objects.annotate(
    no_users=~Exists(User.objects.filter(report=OuterRef('pk')))
).filter(no_users=True)

This can be combined with subqueries. In this example, a Textbook has a number of Versions (ie, version has textbook_id), and a version has a number of Pages (ie, page has version_id). The subquery gets the latest version of each textbook that has pages associated:

subquery = (
    Version.objects
        .filter(
            # OuterRef joins to Version.textbook in outer query below
            textbook=OuterRef('textbook'), 
            # excludes rows with no joined Page records
            page__isnull=False)
        # ordered so [:1] below gets highest (ie, latest) version number
        .order_by('-number').distinct()
)
# Only the Version.ids of the latest versions that have pages returned by the subquery
books = Version.objects.filter(pk=Subquery(subquery.values('pk')[:1])).distinct()

To return rows that have a join to one or both of two tables, use Q objects (Page and TextMarkup both have nullable foreign keys joining to File):

from django.db.models import Q

File.objects.filter(Q(page__isnull=False) | Q(textmarkup__isnull=False).distinct()