id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
27764	Running migration that deletes child model (in multi-table inheritance) backwards results in an error	Alexandru Mărășteanu	nobody	"1. Create two models where one of them inherits the other:
{{{
class Foo(models.Model):
    test = models.CharField(max_length=256)


class Bar(Foo):
    pass
}}}

2. Generate and run migrations.

`migrations/0001_initial.py` is:
{{{
...
    operations = [
        migrations.CreateModel(
            name='Foo',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('test', models.CharField(max_length=256)),
            ],
        ),
        migrations.CreateModel(
            name='Bar',
            fields=[
                ('foo_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='foo.Foo')),
            ],
            bases=('foo.foo',),
        ),
    ]
...
}}}

3. Delete the child model (`Bar`)

4. Generate and run migrations again.

`migrations/0002_auto_20170123_1212.py` is:
{{{
...
        migrations.RemoveField(
            model_name='bar',
            name='foo_ptr',
        ),
        migrations.DeleteModel(
            name='Bar',
        ),
...
}}}

5. Run migrations backwards
{{{
$ python manage.py migrate foo zero
Operations to perform:
  Unapply all migrations: foo
Running migrations:
  Rendering model states... DONE
  Unapplying foo.0002_auto_20170123_1212...Traceback (most recent call last):
  File ""manage.py"", line 10, in <module>
    execute_from_command_line(sys.argv)
  File ""/djbug/lib/python2.7/site-packages/django/core/management/__init__.py"", line 367, in execute_from_command_line
    utility.execute()
  File ""/djbug/lib/python2.7/site-packages/django/core/management/__init__.py"", line 359, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ""/djbug/lib/python2.7/site-packages/django/core/management/base.py"", line 294, in run_from_argv
    self.execute(*args, **cmd_options)
  File ""/djbug/lib/python2.7/site-packages/django/core/management/base.py"", line 345, in execute
    output = self.handle(*args, **options)
  File ""/djbug/lib/python2.7/site-packages/django/core/management/commands/migrate.py"", line 204, in handle
    fake_initial=fake_initial,
  File ""/djbug/lib/python2.7/site-packages/django/db/migrations/executor.py"", line 119, in migrate
    state = self._migrate_all_backwards(plan, full_plan, fake=fake)
  File ""/djbug/lib/python2.7/site-packages/django/db/migrations/executor.py"", line 194, in _migrate_all_backwards
    self.unapply_migration(states[migration], migration, fake=fake)
  File ""/djbug/lib/python2.7/site-packages/django/db/migrations/executor.py"", line 264, in unapply_migration
    state = migration.unapply(state, schema_editor)
  File ""/djbug/lib/python2.7/site-packages/django/db/migrations/migration.py"", line 178, in unapply
    operation.database_backwards(self.app_label, schema_editor, from_state, to_state)
  File ""/djbug/lib/python2.7/site-packages/django/db/migrations/operations/fields.py"", line 153, in database_backwards
    schema_editor.add_field(from_model, to_model._meta.get_field(self.name))
  File ""/djbug/lib/python2.7/site-packages/django/db/backends/postgresql/schema.py"", line 21, in add_field
    super(DatabaseSchemaEditor, self).add_field(model, field)
  File ""/djbug/lib/python2.7/site-packages/django/db/backends/base/schema.py"", line 409, in add_field
    self.execute(sql, params)
  File ""/djbug/lib/python2.7/site-packages/django/db/backends/base/schema.py"", line 112, in execute
    cursor.execute(sql, params)
  File ""/djbug/lib/python2.7/site-packages/django/db/backends/utils.py"", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File ""/djbug/lib/python2.7/site-packages/django/db/backends/utils.py"", line 64, in execute
    return self.cursor.execute(sql, params)
  File ""/djbug/lib/python2.7/site-packages/django/db/utils.py"", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File ""/djbug/lib/python2.7/site-packages/django/db/backends/utils.py"", line 64, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: column ""foo_ptr_id"" of relation ""foo_bar"" already exists
}}}

Verified with Django (1.9.6, 1.9.12 and 1.10.5) and psycopg2 (2.5.2) on macOS 10.12.2 with Python 2.7.12, PostgreSQL 9.5.4."	Bug	closed	Migrations	1.10	Normal				Unreviewed	0	0	0	0	0	0