Running migration that deletes child model (in multi-table inheritance) backwards results in an error

[Alexandru Mărășteanu]

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.