Renaming field and providing prior field name to db_column should be an SQL noop

[Jacob Walls]

Renaming a field and setting the prior implicit field name as the db_column to avoid db operations creates a migration emitting unnecessary SQL. Similar to #31826, which handled a very similar scenario but where there is no field rename, I would expect a SQL noop. I tested with SQLite and MySQL 5.7.31.

class Apple(models.Model):
    core = models.BooleanField()

class Apple(models.Model):
    core_renamed = models.BooleanField(db_column='core')

Was apple.core renamed to apple.core_renamed (a BooleanField)? [y/N] y
Migrations for 'renamez':
  renamez/migrations/0002_rename_core_apple_core_renamed_and_more.py
    - Rename field core on apple to core_renamed
    - Alter field core_renamed on apple

python manage.py sqlmigrate renamez 0002 showing unnecessary SQL:

BEGIN;
--
-- Rename field core on apple to core_renamed
--
ALTER TABLE "renamez_apple" RENAME COLUMN "core" TO "core_renamed";
--
-- Alter field core_renamed on apple
--
ALTER TABLE "renamez_apple" RENAME COLUMN "core_renamed" TO "core";
COMMIT;

Without renaming the field, follow the same flow and get an AlterField migration without SQL, which is what #31826 intended:

BEGIN;
--
-- Alter field core on apple
--
COMMIT;

[Simon Charette]

I think this is due to how the auto-detector ​generates field renames before field alterations. I assume the issue goes away if you swap the order of operations in your migration?

​As this test exemplifies the RenameField operation happens before the AlterField operation does so we'd need to adjust generate_renamed_fields to add an AlterField and prevents generate_altered_fields for doing so when this happens.

[Jacob Walls]

Thanks for having a look. I see now the scope of #31826 was just for flows where the field is not renamed. So that makes this ticket a request to extend this to field renames, which looks like was discussed as 3 and 4 here.

I assume the issue goes away if you swap the order of operations in your migration?

If I switch the order to have AlterField followed by RenameField, FieldDoesNotExist is raised when migrating. These are the operations:

    operations = [
        migrations.RenameField(
            model_name='apple',
            old_name='core',
            new_name='core_renamed',
        ),
        migrations.AlterField(
            model_name='apple',
            name='core_renamed',
            field=models.BooleanField(db_column='core'),
        ),
    ]