can't revert migration with index_together with one field

[rm_]

With django 1.8.14 and postgresql 9.4 i added a composite index with one field (lame, sorry!) because the field is from an abstract model and it seemed the cleanest way to handle that:

+    class Meta:
+        index_together = ['owner']

The migrations got created and applied finely:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('foobar', '0002_auto_20160315_1805'),
    ]

    operations = [
        migrations.AlterIndexTogether(
            name='answer',
            index_together=set([('owner',)]),
        ),
    ]

Problem is i can't revert the migration:

  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/venv/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 354, in execute_from_command_line
    utility.execute()
  File "/venv/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 "/venv/local/lib/python2.7/site-packages/django/core/management/base.py", line 394, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/venv/local/lib/python2.7/site-packages/django/core/management/base.py", line 445, in execute
    output = self.handle(*args, **options)
  File "/venv/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 "/venv/local/lib/python2.7/site-packages/django/db/migrations/executor.py", line 112, in migrate
    self.unapply_migration(states[migration], migration, fake=fake)
  File "/venv/local/lib/python2.7/site-packages/django/db/migrations/executor.py", line 168, in unapply_migration
    state = migration.unapply(state, schema_editor)
  File "/venv/local/lib/python2.7/site-packages/django/db/migrations/migration.py", line 162, in unapply
    operation.database_backwards(self.app_label, schema_editor, from_state, to_state)
  File "/venv/local/lib/python2.7/site-packages/django/db/migrations/operations/models.py", line 415, in database_backwards
    return self.database_forwards(app_label, schema_editor, from_state, to_state)
  File "/venv/local/lib/python2.7/site-packages/django/db/migrations/operations/models.py", line 411, in database_forwards
    getattr(new_model._meta, self.option_name, set()),
  File "/venv/local/lib/python2.7/site-packages/django/db/backends/base/schema.py", line 336, in alter_index_together
    self._delete_composed_index(model, fields, {'index': True}, self.sql_delete_index)
  File "venv/local/lib/python2.7/site-packages/django/db/backends/base/schema.py", line 349, in _delete_composed_index
    ", ".join(columns),
ValueError: Found wrong number (2) of constraints for cicero_academy_answeracademy(owner_id)

So the easy way to fix this is to don't permit te creation of a migration with less than two fields. OTOH it would be quite cool to use the non composite way of removing an index if the constraints are not enough. Whatever you decide i can spend some time on the fix.

[Tim Graham]

Meta.index_together is headed toward deprecation in favor of Meta.indexes (new feature in 1.11). Can the issue be reproduce there? If not, I think we can close this as wontfix.

[rm_]

If Meta.indexes is supposed to be a generic way to add indexes, then the single field for an index is a normal case and there should be no issue.