Removing unique_together creates invalid migration

[Robin Elvin]

I had a model which had a unique_together constraint defined. Now I wish to remove this so the unique_together was removed from the model and a migration created. When applying the migration the following error occurred:

  Applying main.0091_auto_20170316_1046...Traceback (most recent call last):
  File "manage.py", line 8, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 354, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 346, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/base.py", line 394, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/base.py", line 445, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/commands/migrate.py", line 222, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/executor.py", line 110, in migrate
    self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/executor.py", line 148, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/migration.py", line 115, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/usr/local/lib/python2.7/site-packages/django/db/migrations/operations/models.py", line 359, in database_forwards
    getattr(new_model._meta, self.option_name, set()),
  File "/usr/local/lib/python2.7/site-packages/django/db/backends/base/schema.py", line 320, in alter_unique_together
    self._delete_composed_index(model, fields, {'unique': True}, self.sql_delete_unique)
  File "/usr/local/lib/python2.7/site-packages/django/db/backends/mysql/schema.py", line 80, in _delete_composed_index
    return super(DatabaseSchemaEditor, self)._delete_composed_index(model, fields, *args)
  File "/usr/local/lib/python2.7/site-packages/django/db/backends/base/schema.py", line 349, in _delete_composed_index
    ", ".join(columns),
ValueError: Found wrong number (0) of constraints for main_projectdrivefile(name, directory_id)

The model's Meta before starting which was subsequently removed prior to creating the migration:

    class Meta:
        unique_together = ('name', 'directory')

The migration created looks like this:

class Migration(migrations.Migration):

    dependencies = [
        ('main', '0090_auto_20170314_1033'),
    ]

    operations = [
        migrations.AlterUniqueTogether(
            name='projectdrivefile',
            unique_together=set([]),
        ),
    ]

Steps to reproduce:

1. Create a model with a unique_together constraint
2. Create a migration for this model with makemigrations
3. Apply migration
4. Remove unique_together completely
5. Create a migration for this model with makemigrations
6. Apply migration

Note: There appears to have been at least 1 report of this bug previously but the reporter could not recreate so the bug was closed.