Migrations that rename subclasses cause problems with relations

[Matthew Wilkes]

On a Django 1.7.4 site (and 1.7.x), if you have a model that is the destination of a relation and subclass that model then migrations for the subclass incorrectly cause the relations aimed at the parent to be repointed. The generated migrations look like noops, as the models in question aren't actually changed. I had this issue today on a DjangoCMS site, causing migrations to be generated in my django-cms egg.

For example:

class Author(models.Model):
    real_name = models.CharField('name', max_length=50)

class Pseudonym(Author):
    pseudonym = models.CharField('name', max_length=50)

class Book(models.Model):
    author = models.ForeignKey(Author)

If the class Pseudonym is renamed then later migrations will always attempt to reset the author field on book.

This is already fixed on 1.8 and master.

[Matthew Wilkes]

Please see ​https://github.com/django/django/pull/4155 for a fix

[Matthew Wilkes]

Mark as having patch.

[Tim Graham]

Bisected the fix on master/1.8 to b29f3b51204d53c1c8745966476543d068c173a2.

[Matthew Wilkes]

I'm sorry, are you asking me to bisect the current state or saying though found that commit from bisecting?

Either way, that's right. That PR fixes the issue and it was present on master immediately prior to it being merged. I know this because that's where I originally fixed it, and had to change my PR to target 1.7 as MarkusH's PR landed while I was working in it.

He advised that the changes on 1.8 would not be back ported to the 1.7 line.

[Tim Graham]

Sorry, I had a typo in my earlier comment; edited to fix it and reviewed your PR.

[Tim Graham]

Looks good to me, hopefully a migrations expert can give a final +1.

[Matthew Wilkes]

I have made the requested changes and updated the pull request. Sorry for the slow!

[Tim Graham <timograham@…>]

In d0607a7e:

[1.7.x] Fixed #24354 -- Prevented repointing of relations on superclasses when migrating a subclass's name change

[Tim Graham <timograham@…>]

In ae87ad0:

Refs #24354 -- Prevented repointing of relations on superclasses when migrating a subclass's name change

Forwardport of test and release note from stable/1.7.x

[Tim Graham <timograham@…>]

In 22a67d22:

[1.8.x] Refs #24354 -- Prevented repointing of relations on superclasses when migrating a subclass's name change

Forwardport of test and release note from stable/1.7.x

Backport of ae87ad005f7b62f5fa5a29ef07443fa1bbb9baf0 from master

[Markus Holtermann <info@…>]

In e4b7daec:

Refs #24354 -- Prevented repointing of relations on superclasses when migrating a subclass's name change

The issue was hidden on 1.8+ until #24573 due to a bug inside the model
reloading process.

Forwardport of patch from ae87ad005f7b62f5fa5a29ef07443fa1bbb9baf0

[Markus Holtermann <info@…>]

In 697317f:

[1.8.x] Refs #24354 -- Prevented repointing of relations on superclasses when migrating a subclass's name change

The issue was hidden on 1.8+ until #24573 due to a bug inside the model
reloading process.

Forwardport of patch from ae87ad005f7b62f5fa5a29ef07443fa1bbb9baf0