Django models - how to filter number of ForeignKey objects

The question and selected answer are from 2008 and since then this functionality has been integrated into the django framework. Since this is a top google hit for "django filter foreign key count" I'd like to add an easier solution with a recent django version using Aggregation.

from django.db.models import Countcats = A.objects.annotate(num_b=Count('b')).filter(num_b__lt=2)

In my case I had to take this concept a step further. My "B" object had a boolean field called is_available, and I only wanted to return A objects who had more than 0 B objects with is_available set to True.

A.objects.filter(B__is_available=True).annotate(num_b=Count('b')).filter(num_b__gt=0).order_by('-num_items')

Sounds like a job for extra.

A.objects.extra(    select={        'b_count': 'SELECT COUNT(*) FROM yourapp_b WHERE yourapp_b.a_id = yourapp_a.id',    },    where=['b_count < 2'])

If the B count is something you often need as a filtering or ordering criterion, or needs to be displayed on list views, you could consider denormalisation by adding a b_count field to your A model and using signals to update it when a B is added or deleted:

from django.db import connection, transactionfrom django.db.models.signals import post_delete, post_savedef update_b_count(instance, **kwargs):    """    Updates the B count for the A related to the given B.    """    if not kwargs.get('created', True) or kwargs.get('raw', False):        return    cursor = connection.cursor()    cursor.execute(        'UPDATE yourapp_a SET b_count = ('            'SELECT COUNT(*) FROM yourapp_b '            'WHERE yourapp_b.a_id = yourapp_a.id'        ') '        'WHERE id = %s', [instance.a_id])    transaction.commit_unless_managed()post_save.connect(update_b_count, sender=B)post_delete.connect(update_b_count, sender=B)

Another solution would be to manage a status flag on the A object when you're adding or removing a related B.

B.objects.create(a=some_a)if some_a.hidden and some_a.b_set.count() > 1:    A.objects.filter(id=some_a.id).update(hidden=False)...some_a = b.asome_b.delete()if not some_a.hidden and some_a.b_set.count() < 2:    A.objects.filter(id=some_a.id).update(hidden=True)

I'd recommend modifying your design to include some status field on A.

The issue is one of "why?" Why does A have < 2 B's and why does A have >= 2 B's. Is it because user's didn't enter something? Or is because they tried and their input had errors. Or is it because the < 2 rule doesn't apply in this case.

Using presence or absence of a Foreign Key limits the meaning to -- well -- present or absent. You don't have any way to represent "why?"

Also, you have the following option

[ a for a in A.objects.all() if a.b_set.count() < 2 ]

This can be pricey because it does fetch all the A's rather than force the database to do the work.

Edit: From the comment "would require me to watch for user join / user leaving the pool events".

You don't "watch" anything -- you provide an API which does what you need. That's the central benefit of the Django model. Here's one way, with explict methods in the A class.

class A( models.Model ):    ....    def addB( self, b ):        self.b_set.add( b )        self.changeFlags()    def removeB( self, b ):        self.b_set.remove( b )        self.changeFlags()    def changeFlags( self ):        if self.b_set.count() < 2: self.show= NotYet        else: self.show= ShowNow

You can also define a special Manager for this, and replace the default b_set Manager with your manager that counts references and updates A.