Re: OneToOneField usage

On May 13, 2010, at 10:29 AM, TallFurryMan wrote:

> Hello Django users,
>
> Is there a particular reason why using a related OneToOneField raises
> DoesNotExist instead of returning None?

Any query you make that is supposed to return one or more instances, that instead cannot find any results, returns a DoesNotExist.
>
> | class Person( Model ):
> | pass
> |
> | class Pet( Model ):
> | owner = OneToOneField( Person )
> |
> | # Assuming "joe" exists as a Person
> | >>> kitty = joe.pet
> | DoesNotExist: Pet matching query does not exist.

Your process is wrong. You are assuming "joe" exists. But your syntax, "kitty = joe.pet", assumes that "pet" already exists. You are assigning to "kitty" a reference to the object named by "joe.pet". If "joe.pet" does not exist, well, you get a DoesNotExist.
>
> SingleRelatedObjectDescriptor is expecting the related model to exist.
> OneToOneField being a subclass of ForeignKey, my first attempt was:
>
> | >>> kitty = joe.pet.get_or_create()
> | DoesNotExist: Pet matching query does not exist.

This should be:

kitty = Pet.objects.get_or_create(user=joe)

>
> It's the getter of "joe.pet" which fails. Is it that OneToOneField is
> not popular enough to receive such an implementation, or is there a
> deeper philosophical reason I don't see?

No reason, just that your syntax is incorrect.

---Peter Herndon

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