Generated migration orders Add/Remove Field incorrectly, causing OperationalError

[Ed Morley]

Replacing an integer field with a foreign key of the same name, results in an OperationalError when creating/applying the migration.

This affects Django master + v1.11.10, and both the SQLite and MySQL backends (others not tested).

STR:

1. Git clone ​https://github.com/edmorley/django-migration-int-to-fk-testcase
2. pip install https://github.com/django/django/archive/master.zip
3. ./manage.py migrate
4. cp testapp/models_new.py testapp/models.py
5. ./manage.py makemigrations --name broken_migration
6. ./manage.py migrate

Expected:

New migration is created/applied successfully, which converts from the ​original model to the ​new model.

Actual:

The new 0002_broken_migration.py migration incorrectly lists the AddField operation before the RemoveField operation...

    operations = [
        migrations.AddField(
            model_name='bar',
            name='foo',
            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.CASCADE, to='testapp.Foo'),
        ),
        migrations.RemoveField(
            model_name='bar',
            name='foo_id',
        ),
        migrations.AlterUniqueTogether(
            name='bar',
            unique_together={('name', 'foo')},
        ),
    ]

Which results in an exception at step 6...

$ ./manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, sessions, testapp
Running migrations:
  Applying testapp.0002_broken_migration...Traceback (most recent call last):
  File ".../site-packages/django/db/backends/utils.py", line 83, in _execute
    return self.cursor.execute(sql)
  File ".../site-packages/django/db/backends/sqlite3/base.py", line 290, in execute
    return Database.Cursor.execute(self, query)
sqlite3.OperationalError: duplicate column name: foo_id

Additional notes:

• Without the unique_together on model Bar, the bug does not occur.
• This affects both the SQLite backend and the MySQL backend (others not tested).
• At time of testing, django master was at revision 6d794fb76212bb8a62fe2cd97cff173054e1c626.
• The above was using Python 3.6.2.