GenericForeignKey.get_content_type fails if `object` or `id` evaluate to False. Should use 'is not None' test.

[Ben]

contenttypes.GenericForeignKey.get_content_type is used to find the content type of an object. If the object evaluates to False, then an error is raised. This is because 'if obj:' is used, rather than 'if obj is not None:'. The same problem exists for the 'id' argument, and I believe also in the __get__ method.

[Russell Keith-Magee]

Two quick points:

1) Please generate patches relative to the root of the source tree
2) Please provide a test case, not just a patch. Under what conditions will the object be false? Ideally, this should be in the form of a regression test integrated into Django's test suite, but if you can't work out how to do that, a code sample will do.

Accepting because I think I can spot the problem case (getting the content type for an unsaved object)

[Ben]

Patch with tests included

[Ben]

I've included a new patch with a regression test included. The object will be false if it has nonzero is defined and returns False, or if len is defined and returns 0. Either of these should be possible without generic objects failing. The bug would also cause problems if a generic object was used with content_type_id=0.

[Łukasz Rekucki]

Patch no longer cleanly applies to trunk.

[Ramiro Morales]

#14325 and #19494 were duplicates.

[Ramiro Morales]

See also #13137.

[Ramiro Morales <cramm0@…>]

In 04d9730b127c689b8eda01cbc913efa6e2eb230b:

Fixed #13085 -- Don't fail on creation of model with GFK to a model with len() returning zero.

Also, according to the comments on the ticket and its duplicates, added
tests execising saving an instance of a model with a GFK to:

• An unsaved object -- This actually doesn't generate the same failure but another ORM-level exception. The test verifies it's the case.

• An instance of a model with a nonzero() method thant returns False for it. This doesn't fail because that code path isn't executed.

• An instance of a model with a CharField PK and an empty value for it. This doesn't fail.

[GitHub <noreply@…>]

In 1ed8ca43:

Refs #13085 -- Removed unnecessary ManyToManyFields from generic_relations_regress test models.