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 (i.e. python migrate APP zero)

   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.