Document that GenericForeignKey returns None when referring to nonexistent pk

[Sjoerd Job Postmus]

We have a model as follows.

class RemoteIdentifierToLocal(models.Model):
    remote_id = models.UUIDField(primary_key=True)

    local_object_content_type = models.ForeignKey(ContentType)
    local_object_object_id = models.PositiveIntegerField()
    local_object = GenericForeignKey(ct_field='local_object_content_type', fk_field='local_object_object_id')

which is used as follows:

if guid is not None:
    local_object = RemoteIdentifierToLocal.objects.get(remote_id=guid).local_object

To my surprise, we found a case where guid was not-None, and local_object was None.

This was traced back to the implementation of GenericForeignKey containing a try/except-pass:

​https://github.com/django/django/blob/master/django/contrib/contenttypes/fields.py#L241 (now)
​https://github.com/django/django/commit/bca5327b21eb2e3ee18292cbe532d6d0071201d8#diff-cc83843e623c1ab07a24317073330058R62 (initial commit, 11 years ago)

Now, apparently this has been this way for 11 years already, so must be the "correct" behaviour. However I have not found any documentation of this fact, and would (myself) expect the attribute-access to *do* raise an exception or warning of some sort.

[Tim Graham]

There's ​one assertion in Django's test suite that fails with the exception catching removed. I doubt it's appropriate to raise a warning for properly working code (assuming the test represents a valid use case -- I didn't study it in much detail). Accepting at least as a documentation enhancement.

[Harshit Jain]

I would like to document this. Can you tell me where do I need to add this?

[Harshit Jain]

The Documentation has been added. Here is the link to the pull request - ​https://github.com/django/django/pull/9181.

[Tim Graham <timograham@…>]

In f9a01d40:

[2.0.x] Fixed #28613 -- Doc'd the return value for GenericForeignKey when the related object is deleted.

Backport of 1a82fc245eb8ac4b131ec02a6ac3e112deb8d5a6 from master

[Tim Graham <timograham@…>]

In 1a82fc2:

Fixed #28613 -- Doc'd the return value for GenericForeignKey when the related object is deleted.