It's impossible to revert migrations

[yurasavin]

Describe the bug
I've faced with problem that it's impossible to revert migrations because migration 0006_alter_application_client_secret has no revert action

To Reproduce

1. Apply migrations python manage.py migrate oauth2_provider
2. Try to revert migrations python manage.py migrate oauth2_provider zero

Expected behavior
Migrations are reverted

Version
2.2.0

• I have tested with the latest published release and it's still a problem.
• I have tested with the master branch and it's still a problem.

Additional context

[yurasavin]

This issue is not a big deal so I've created a PR with fix #1296

I hope I haven't break any rules for the issues and PR's

[n2ygk]

Per #1093 (see CHANGELOG for 2.0.0 migration 0006 is not reversible because it performs a one-way hash of any cleartext secrets.

I'm not sure what the right answer is here.

[yurasavin]

Oh, I see now. OK we can't revert migrations without loosing the data, but how about to check in forwards_func if data has already hashed by prefix pbkdf2_sha256$ and skip hashing in case we reapply the migration?

[n2ygk]

That's already happening in https://github.com/jazzband/django-oauth-toolkit/blob/master/oauth2_provider/models.py

[yura-savin]

I mean in some cases you need to roll back migrations, I've faced this situation in my development. I don't care about is my secrets hashed or not in this case, I just need to roll back this migration from django migrations table, but without revert function it's become unpossible by applying manage.py migrate oauth2_provider 0005 for example.

[n2ygk]

Agreed. Maybe add the reverse migration to 0006 like you did in #1296 but have it print a warning or something.

[dopry]

This was resolved in #1296. However I would like to say that reverse migrations are not something we can guaranteed. I feel we should avoid allowing reversing where it would lead to a broken site. Best practice is to test migrations in UAT environments, and to back up production data before apply migrations to minimize data loss in case an unreversible migration is applied and someone must revert their code.

If someone were to reverse over migration 006 and revert the code, then auth would fail since client_id lookups would no longer work.