Deriving from abstract model with foreign key to model in same app broken on Django 1.9

[Karl Hobley]

As of Django 1.9, it is no longer possible to derive a new model from abstract model in another app if that abstract model has a local foreign key to a model in the same app.

See example project here: ​https://github.com/kaedroho/djangobugtest

For example, I have two apps: app1 and app2

app1 contains a concrete model and an abstract model. The abstract model has a foreign key to the concrete one (note the Foreign key is defined as a string and doesn't include the app label):

class ConcreteModel(models.Model):
    pass


class AbstractModel(models.Model):
    foreign_key = models.ForeignKey('ConcreteModel')

    class Meta:
        abstract = True

In app2, we have another model that derives from the abstract model in app 1

class DerivedModel(app1.AbstractModel):
    pass

When running makemigrations, the following system check error occurs:

app2.DerivedModel.foreign_key: (fields.E300) Field defines a relation with model 'ConcreteModel', which is either not installed, or is abstract.

For apps upgrading to Django 1.9 and already have migrations, the following error occurs when running tests (after the database has been initialised but before the first test runs. It looks like it's happening while serialising the database):

ValueError: Related model u'ConcreteModel' cannot be resolved

Traceback from an actual site: ​https://gist.github.com/kaedroho/01b29332308018abfa29

This issue only occurs when the ForeignKey is defined with a string that doesn't contain the app label. So referencing the model class directly or using "app_label.ModelName" works.

[Karl Hobley]

git-bisect points here: ​https://github.com/django/django/commit/9239f1dda7b94f53d21efb8b5e4d056e24f4e906
Introduced in: ​https://github.com/django/django/pull/4115

[Tim Graham]

Not sure if the issue can/should be fixed, but we could at least document the requirement and backwards incompatibility.

[Simon Charette]

I think we should try fixing this. It can lead to confusing errors if a second ConcreteModel is defined in app2 for example. I'll see if I can come up with a non-invasive patch.

[Tim Graham]

I guess there's a small design decision about whether or not it could possibly be a useful "feature" to have the option of using the model name only in the ForeignKey to be able to refer to a local model in the app in which the abstract model is actually used and to require an app_label in the ForeignKey otherwise?

[Simon Charette]

Indeed, I've been trying to reach Carl to talk about it but I guess I should simply write to @developpers.

If we were to consider this change as a feature I guess we should go through a deprecation period.

[Simon Charette]

Posted to @developpers: ​https://groups.google.com/d/msg/django-developers/jRut8aYEet0/IrSiXwK5EgAJ

[Simon Charette]

​PR based on input from Markus, Shai and Aymeric.

[Simon Charette <charette.s@…>]

In bc7d201b:

Fixed #25858 -- Bound abstract model application relative relationships.

Thanks to Karl Hobley for the report and Markus, Shai, Aymeric for their input
and Tim for the review.

[Simon Charette <charette.s@…>]

In 27ef6403:

[1.9.x] Fixed #25858 -- Bound abstract model application relative relationships.

Thanks to Karl Hobley for the report and Markus, Shai, Aymeric for their input
and Tim for the review.

Backport of bc7d201bdbaeac14a49f51a9ef292d6312b4c45e from master

[Simon Charette]

In order to fix #26186 the patch for this ticket is planed to be reverted in Django 1.9.3.

The suggested approach for the issue reported here is to bound your app relative reference to an application (app_label.ConcreteModel in your reported case) because what we initially thought as a new behavior introduced in Django 1.9 has been possible since at least Django 1.2 given the appropriate setup.

In order to support both use cases it was decided that app relative lazy relationships should be resolved against their concrete model's app_label and an explicit app_label should be added to the lazy relationship if the abstract model was meant to be used in a foreign application while maintaining a relationship to a specific application's model.

[Simon Charette <charette.s@…>]

In 0223e213:

Fixed #26186 -- Documented how app relative relationships of abstract models behave.

This partially reverts commit bc7d201bdbaeac14a49f51a9ef292d6312b4c45e.

Thanks Tim for the review.

Refs #25858.

[Simon Charette <charette.s@…>]

In 48cf7516:

[1.9.x] Fixed #26186 -- Documented how app relative relationships of abstract models behave.

This partially reverts commit bc7d201bdbaeac14a49f51a9ef292d6312b4c45e.

Thanks Tim for the review.

Refs #25858.

Backport of 0223e213dd690b6b6e0669f836a20efb10998c83 from master