GenericForeignKey attributes create new instances on every access

[Morgan Wahl]

Given these models:

class OtherSuper(models.Model):
    pass


class OtherSub(OtherSuper):
    pass


class Ref(models.Model):
    obj_type = models.ForeignKey(ContentType, on_delete=models.PROTECT)
    obj_id = models.CharField(max_length=255)
    obj = GenericForeignKey('obj_type', 'obj_id')

I get this behavior:

In [1]: ref = Ref.objects.create(obj=OtherSub.objects.create())

In [2]: id(ref.obj) == id(ref.obj)
Out[2]: True

In [3]: ref.refresh_from_db()

In [4]: id(ref.obj) == id(ref.obj)
Out[4]: False

Each time ref.obj is accessed, a new instance is created for its value. This is a problem, since doing something like ref.obj.field = 1; ref.obj.save() won't actually update the field in the database. This only happens when the referenced object is an instance of a model that subclasses another model. (So, it wouldn't happen if referencing OtherSuper in the models above.) The refresh_from_db() call is also necessary to reproduce this in a simple test like the above; it happens with any instance created from an existing DB record.

I've written a regression test against stable/1.10.x . I'll attach a patch.

I discovered this because I have code that does the above (changes a field on the related model and calls save). I call this a regression because it works correctly on 1.9.

I'm not sure what the underly bug is; I looked at the diff in contenttypes between 1.9 and 1.10, and there are more than a few changes. Hopefully someone who understands the GenericForeignKey implementation can figure this out.

[Morgan Wahl]

patch with regression test