CREATE INDEX-related OperationalError on migrating renamed models with colliding field names

[Chris Woytowitz]

This one's a bit of an edge case, but I ran into it trying to refactor some models on my moderately large work project.

Let's say I have a Django 1.7 app called sample:

1. Create a model Bar.
2. Create a model Foo with bar = models.ForeignKey(Bar, blank=True, null=True)
3. makemigrations (migration 0001)
4. Rename Foo to OldFoo.
5. makemigrations, say yes to the rename prompt (migration 0002)
6. Create new model Foo, which also has bar = models.ForeignKey(Bar, blank=True, null=True)
7. makemigrations (migration 0003)
8. migrate

When migrate hits 0003, it throws this error:
django.db.utils.OperationalError: index sample_foo_4350f7d0 already exists

You may notice that sqlmigrate sample 0001 and sqlmigrate sample 0003 have the same line in both of them:
CREATE INDEX sample_foo_4350f7d0 ON "sample_foo" ("bar_id");

In my production case, I actually had no problems on servers, because South had created the index with a different name. When I renamed the models and added a field, the new index did not collide with the old one. However, our test suite started failing, because it would run the migrations from the ground up, exposing the above bug.

I haven't decided on a workaround yet, but I thought I'd bring this to your attention. I might have to inject a migration that renames the index created in 0001, or something to that effect.

The attached test case has a project dj17test in the state after having performed all of the above steps.