Migration does not drop old M2M through-table when ManyToManyField target model changes

[Johanan Oppong Amoateng]

When changing the target model of a ManyToManyField, the migration autodetector generates only an AlterField operation. Django does not create a RemoveField + AddField sequence, and the old automatically generated through-table is left in place.

This results in the database schema no longer matching the model definitions. In my case, the stale through-table caused inconsistent M2M behaviour and errors when interacting with the relationship. No data records were lost, but the schema was incorrect until manually fixed.

This behaviour appears inconsistent with how Django handles other destructive schema changes.

[Jacob Walls]

Thanks, replicated. Patch looks straightforward, would you be willing to submit a PR and extrapolate from similar existing test cases?

django/db/migrations/autodetector.py

@@ -1339 +1339 @@
             if old_field_dec != new_field_dec and old_field_name == field_name:
                 both_m2m = old_field.many_to_many and new_field.many_to_many
                 neither_m2m = not old_field.many_to_many and not new_field.many_to_many
-                if both_m2m or neither_m2m:
+                target_changed = both_m2m and new_field_dec[2]["to"] != old_field_dec[2]["to"]
+                if (both_m2m or neither_m2m) and not target_changed:
                     # Either both fields are m2m or neither is
                     preserve_default = True
                     if (

[Johanan Oppong Amoateng]

Yes please i will submit a patch for this

[Johanan Oppong Amoateng]

​https://github.com/django/django/pull/20395

[Jacob Walls <jacobtylerwalls@…>]

In 33a0a6f0:

Fixed #36791 -- Made MigrationAutodetector recreate through table when m2m target model changes.

Co-Authored-By: Jacob Walls <38668450+jacobtylerwalls@…>
Co-Authored-By: Clifford Gama <cliffygamy@…>