ForeignKeyField Renaming leads Remove and AddField instead of RenameField Migration

[Taqi Abbas]

Given this field in the model:

class Answer(models.Model):
	question = models.ForeignKey(question_models.Question, on_delete=models.CASCADE, null=True, blank=True)
	...

I changed this to:

class Answer(AbstractModel):
	_question = models.ForeignKey(question_models.Question, db_column='question_id', on_delete=models.CASCADE, null=True, blank=True)
	...

the resulting migration was:

	...
        migrations.RemoveField(
            model_name='answer',
            name='question',
        ),
        migrations.AddField(
            model_name='answer',
            name='_question',
            field=models.ForeignKey(blank=True, db_column='question_id', null=True, on_delete=django.db.models.deletion.CASCADE, to='survey.Question'),
        ),
	...

whereas it should have been the following.

	...
        migrations.RenameField(
            model_name='answer',
            old_name='question',
            new_name='_question',
        ),
	...

But when I manually changed to the above, I get the following error:

ProgrammingError: column survey_answer.question_id does not exist
LINE 1: ...swer"."updated_on", "survey_answer"."is_deleted", "survey_an...
                                                             ^
HINT:  Perhaps you meant to reference the column "survey_answer._question_id".

After this, I ran makemigrations again, without changing any model and this time it made the following:

        migrations.AlterField(
            model_name='answer',
            name='_question',
            field=models.ForeignKey(blank=True, db_column='question_id', null=True, on_delete=django.db.models.deletion.CASCADE, to='survey.Question'),
        ),

The database was PostgreSQL.

[Simon Charette]

The issue here is that the auto-detector won't be able to detect field renames if any other options has been changed in the mean time.

Could you confirm that the following works:

1. Only perform the db_column='question_id' change.
2. Run makemigrations and assert that a migration with an AlterField operation was generated to include the db_column='question_id' change.
3. Change the attribute name from question to _question.
4. Run makemigrations and confirm the field rename. Assert that a migration with a RenameField operation was created.

At this point both migrations should be SQL noops.

I'm accepting on the basis that the auto-detector rename logic could be enhanced to better handle db_column changes by comparing implicit db_column values to implicit ones and prompt for a rename.

[Simon Charette]

Someone at work hit a similar issue so I figured out I'd submit a PR tackling what I suggested above.

​https://github.com/django/django/pull/9829

[Tim Graham <timograham@…>]

In 2156565b:

Fixed #29245 -- Made autodetector treat field renames + db_column addition as RenameField.