select_related does not work with nullable fields unless field is specified.

[Dan Loewenherz]

Say I have the following model:

class Book(models.Model):
      owner = models.ForeignKey(Owner, related_name='books')

Book.objects.select_related() will pull in all owner data. HOWEVER, when the owner field is nullable, no joins occur.

class Book(models.Model):
    owner = models.ForeignKey(Owner, null=True, blank=True, related_name='books')

Running a join in my database involving a nullable field works exactly as would one on a non-nullable field, so I can't really see a good reason for this to happen.

[AndrewIngram]

select_related will follow nullable foreignkeys if you specify the fieldname manually, eg:

Books.objects.select_related('owner')

I'm sure there's a good reason why this doesn't happen by default, but the behaviour is documented.

[Dan Loewenherz]

Thanks for the tip!

In my actual model there are about 6 ForeignKey relationships (all of which are accessed for the request). That means I have to manually specify each one I want to perform a join on, which is a little tedious/unclean.

[Aymeric Augustin]

Digging into the docs and code, there is no explanation of this behavior.

The condition is in django.db.models.query_utils.select_related_descend.

[Malcolm Tredinnick]

A documentation patch is welcome for this (although I thought it was documented originally; may have been a friendly fire casualty). For performance reasons, we will never traverse nullable relations by default, as it requires using left outer joins, instead of merely inner joins, and the former are significantly slower. It is intentional that you can manually tell your code to traverse nullable relations by passing in the field names, so there's no behavioural bug here, just a documentation one.

[Malcolm Tredinnick]

Quoting from the current documentation (​https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.select_related):

Note that, by default, select_related() does not follow foreign keys that have null=True.

So no documentation patch required. The reason it's like this is because nullable joins require an outer join between tables and is sufficiently performance impacting to not do by default.