Document how to use models from other apps in RunPython

[Aymeric Augustin]

A common use case for RunPython (known in South as data migrations) is to port data from one model to another. When you do this across apps, you must add the other app to dependencies. Otherwise you get a moderately helpful:
LookupError: No installed app with label 'people'.

Do you think it's worth documenting?

[Aymeric Augustin]

Here's an interesting variant of this problem.

If you attempt to use a relation that's defined in another app, the relation doesn't exist unless you include that app in your dependencies.

[Tim Graham]

Definitely. I thought this came up before, but I can't find a ticket or any docs.

[Markus Holtermann]

This should definitely be covered in the docs. I only think I can recall to have read somewhere are the few lines I wrote down in my ​Django 1.7: Database Migrations done right blog post.

[Bibhas C Debnath]

I recently faced this when migrating a model which had an FK to a custom user model. The custom user model was added in the migration 0002 of my accounts app. I had to manually add 0002_migration_name to the dependency list for the migration to work, otherwise it'd just complain that it can't find the custom user model. The user models migration was not added to the dependency list automatically. I'll check if I can reproduce this. This happened during the migration that was reported in #24037.

[Tim Graham <timograham@…>]

In b089759:

Fixed #24052 -- Doc'd how to write data migrations with models in multiple apps.

[Tim Graham <timograham@…>]

In 9bb451ae:

[1.7.x] Fixed #24052 -- Doc'd how to write data migrations with models in multiple apps.

Backport of b089759d6025582f36fbea3c4be3855c50b82462 from master

[Tim Graham <timograham@…>]

In 937c8a5a:

[1.8.x] Fixed #24052 -- Doc'd how to write data migrations with models in multiple apps.

Backport of b089759d6025582f36fbea3c4be3855c50b82462 from master