OneToOne fields with null=True raise DoesNotExist exception on related model

[rvdrijst]

Referencing a OneToOneField with null=True (i.e. it's optional) when there is no value set will return None, as expected.
However, referencing the reverse side of the relation does not follow this behavior and raises a DoesNotExist exception.

For example, in the following situation where Shops have optionally a Place (e.g. webshops need not have a physical location):

class Place(models.Model)
    address = models.CharField(max_length=80)
class Shop(models.Model)
    place = models.OneToOneField(Place, null=True)
    name = models.CharField(max_length=50)
    website = models.URLField()

This does work as expected:

>>> s1 = Shop.objects.create(name='Shop', website='shop.com')
>>> print s1.place
None

But this doesn't work as expected:

>>> p1 = Place.objects.create(address='123 somestr')
>>> p1.shop
... [exception stack trace] ...
DoesNotExist: Shop matching query does not exist.

I would expect this to be None when null is allowed on the OneToOneField.

Please correct my if I'm wrong.

I have attached a patch to fix this (checking if null is allowed and returning None or raising the exception appropriately), including tests.

Unfortunately this is slightly backwards incompatible, since someone may currently rely on the exception being thrown.

[rvdrijst]

Patch with fix and tests