How to remove a model definition completely in Django when it previously had foreign keys

[Henrik Ossipoff Hansen]

We're trying to remove a model completely from one of our apps - an operation I think we've done many times without issues, but this time it's causing us some headache. Consider we have two apps:

An app called a with a model A

An app called b with a model B, and this model B has a foreign key to A.

This means the initial Django migration file for app b contains an entry for a foreign key to the string 'a.A'. Let's call this step 1.

Now later, we remove the foreign key on model B. This of course creates a new migration, but the initial migration still has the 'a.A' foreign key definition in it (obviously). Let's call this step 2.

Now the tricky part: we want to remove the whole model A. This in itself creates a migration for app a to that deletes the model, but poses a new problem; when running the full migration history (for example during tests), the initial migration of app b will fail, because it can no longer resolve 'a.A'. Let's call this step 3.

How are people coping with the issue the best? One solution I can think of involves doing a migration squash (and would be sort of easy in our case, since we're actually going to remove the whole app a as well), but it seems a bit extreme. Is there anyone out there with a better and less intrusive solution?

Regards,

Henrik Ossipoff Hansen

[Clara Daia]

Are you getting an error? I think migration dependencies should solve that by themselves, running "step 2" before "step 3". Doesn't Django complain if you try to migrate "step 3" before "step 2"?

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[Henrik Ossipoff Hansen]

Yes, when running tests (which happen on a clean database), the migration history is run all from the initial migrations (by Django itself). At this point, app b's initial migration (which have a reference to a.A) will fail because it cannot find such model:

ValueError: Related model 'a.A' cannot be resolved

[James Bennett]

When you need to remove any piece of code that's been referenced in migrations, generally there's a multi-step process.

For sake of a simple example, let's assume you have Model A in App A, and Model B in App B. And at some point you added a foreign key from Model A to Model B, but now you want to completely remove Model B. The process is:

1. Remove the foreign key from Model A and generate a migration for that. Apply the migration. DO NOT DELETE MODEL B'S CODE YET.

2. Generate a migration with a RemoveModel operation for Model B. DO NOT DELETE MODEL B'S CODE YET.

3. Squash migrations for App A, starting at the point the foreign key to Model B was added, and ending at the point the foreign key to Model B was removed. DO NOT DELETE MODEL B'S CODE YET.

4. Now that only a single migration file references Model B, either remove the AddField/RemoveField operations from it entirely, or replace them with migrations.RunPython.noop operations. DO NOT DELETE MODEL B'S CODE YET.

5. Verify that you can bring up a fresh database correctly with the migrations you now have. DO NOT DELETE MODEL B'S CODE YET.

6. Finally: delete Model B's code from your codebase.

if you referred to this model from multiple apps/models, you will need to do steps 1-5 once for each app/model that had a reference.