Missing reverse migration

It's good to, as a minimum, specify noop in RunPython so the migration can be skipped when going backwards, and even better to specify a function that undoes the migration.

The code in forwards mutates the data, but in order to undo the change then reverse_code is needed.

If reverse_code is missing then when an attempt is made to undo the migration then Django will raise IrreversibleError.

This is a problem because it prevents rolling back the state of the database, which may be needed if a bug is detected after a production deployment and the last good version of the app needs deploying back to production. Without reverse_code, the state of the database will be out of step with the app code.

If we spot this issue in your GitHub pull request we give this advice:

migrations/0002_auto.pymaintainabilityhigh
class Migration(migrations.Migration):
    dependencies = [("core", "0001_initial.py")]
-
    operations = [RunPython(forwards)]
+
    operations = [RunPython(forwards, RunPython.noop)]

It's good to, as a minimum, specify noop in RunPython so the migration can be skipped when going backwards, and even better to specify a function that undoes the migration.

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Configuring this check

Django Doctor will run this check by default. No configuration is needed but the check can be turned on/off using check code missing-reverse-migration in your pyproject.toml file.

Read more about configuring Django Doctor.