I came across a little annoyance in Django today. I found that ForeignKeys that reference 'self', i.e. they point to the same table, always result in a join in a filter.

Take this normal foreign key reference.

   class Customer(models.Models):
       user = models.ForeignKey(User)

   >>> Customer.objects.filter(user__isnull)._as_sql()
   ('SELECT U0."id" FROM "accounts_customer" U0 WHERE U0."customer_id" IS NULL',
())

Now lets look at a version of the customer model with a self reference.

   class Customer(models.Models):
       user = models.ForeignKey(User)
       other_cust = models.ForeignKey('self')

   >>> Customer.objects.filter(user__isnull)._as_sql()
   ('SELECT U0."id" FROM "accounts_customer" U0 LEFT OUTER JOIN "accounts_customer" U1 ON (U0."other_cust_id" = U1."id") WHERE U1."id" IS NULL',
())

Hmm, yuck. That little extra JOIN is going to kill performance if the table is big. Let's do it the right way.

>>> Customer.objects.extra(where=["other_cust_id IS NULL"])
('SELECT U0."id" FROM "accounts_customer" U0 WHERE other_cust_id IS NULL', ())

Ahh, that's better. I don't really like using extra() but in situations like these I'm glad it's there.