AlterField fails when both db_index and unique are specified

[Zachary Voase]

I've put up a proof-of-concept here: ​https://github.com/zacharyvoase/django-migrationbug-poc

If you have a field like:

name = models.CharField(..., db_index=True)

And you add a unique constraint without removing the db_index=True:

name = models.CharField(..., db_index=True, unique=True)

The auto-migration you get from that looks like:

migrations.AlterField(
    model_name='person',
    name='name',
    field=models.CharField(db_index=True, unique=True, max_length=255, blank=True),
)

When Django tries to run this migration you get:

Operations to perform:
  Synchronize unmigrated apps: staticfiles, messages
  Apply all migrations: admin, contenttypes, migration_poc_app, auth, sessions
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
  Installing custom SQL...
Running migrations:
  Rendering model states... DONE
  Applying contenttypes.0001_initial... OK
  Applying auth.0001_initial... OK
  Applying admin.0001_initial... OK
  Applying contenttypes.0002_remove_content_type_name... OK
  Applying auth.0002_alter_permission_name_max_length... OK
  Applying auth.0003_alter_user_email_max_length... OK
  Applying auth.0004_alter_user_username_opts... OK
  Applying auth.0005_alter_user_last_login_null... OK
  Applying auth.0006_require_contenttypes_0002... OK
  Applying migration_poc_app.0001_initial... OK
  Applying migration_poc_app.0002_auto_20160114_2114...Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 354, in execute_from_command_line
    utility.execute()
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 346, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/core/management/base.py", line 394, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/core/management/base.py", line 445, in execute
    output = self.handle(*args, **options)
  File "/Users/zack/src/migration_poc/venv/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 "/Users/zack/src/migration_poc/venv/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 "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/db/migrations/executor.py", line 148, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/Users/zack/src/migration_poc/venv/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 "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/db/migrations/operations/fields.py", line 201, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/db/backends/base/schema.py", line 484, in alter_field
    old_db_params, new_db_params, strict)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/db/backends/postgresql_psycopg2/schema.py", line 113, in _alter_field
    self.execute(like_index_statement)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/db/backends/base/schema.py", line 111, in execute
    cursor.execute(sql, params)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/db/backends/utils.py", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/db/utils.py", line 98, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/Users/zack/src/migration_poc/venv/lib/python2.7/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: relation "migration_poc_app_person_name_32437daa2f5b1fe9_like" already exists

N.B.: If you remove the db_index=True from the AlterField(...) call,
the migration succeeds, even if the field definition in the model still has
db_index=True.

[Simon Charette]

Looks like a duplicate of #26034 which should be fixed with the next release of 1.8.x series (1.8.9).