Generated migration orders RemoveField and AlterUniqueTogether incorrectly, causing FieldDoesNotExist

[Ed Morley]

Removing a field and its corresponding unique_together entry results in a FieldDoesNotExist exception when creating/applying the migration.

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

(See also #29123 which also only reproduces when unique_together is used)

STR:

1. Git clone ​https://github.com/edmorley/django-migration-remove-field-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 RemoveField operation before the AlterUniqueTogether operation...

    operations = [
        migrations.RemoveField(
            model_name='push',
            name='foo',
        ),
        migrations.AlterUniqueTogether(
            name='push',
            unique_together={('repository', 'revision')},
        ),
    ]

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/models/options.py", line 564, in get_field
    return self.fields_map[field_name]
KeyError: 'foo'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  ...
  File ".../site-packages/django/db/backends/base/schema.py", line 286, in create_model
    columns = [model._meta.get_field(field).column for field in fields]
  File ".../site-packages/django/db/backends/base/schema.py", line 286, in <listcomp>
    columns = [model._meta.get_field(field).column for field in fields]
  File ".../site-packages/django/db/models/options.py", line 566, in get_field
    raise FieldDoesNotExist("%s has no field named '%s'" % (self.object_name, field_name))
django.core.exceptions.FieldDoesNotExist: Push has no field named 'foo'

Additional notes:

• If the ('repository', 'revision') entry in unique_together is omitted from both the original and new models, then 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.

[Tim Graham]

Duplicate of #28862 I think.