Django migration for converting model to django-polymorphic child model

000x_convert_foo_to_polymorphic_child.py

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

from django.db import migrations, models
from django.db.models import F


def seed_baseitem(apps, schema_editor):
    """ For each Foo, create a BaseItem with the same primary key value and
    the appropriate `polymorphic_ctype` value.  This will be used by a new
    `Foo.baseitem_ptr` foreign key so that each `Foo` instance can retain
    its ID after `Foo` is converted to a polymorphic child class
    """
    Foo = apps.get_model('myapp', 'Foo')
    BaseItem = apps.get_model('myapp', 'BaseItem')
    ContentType = apps.get_model('contenttypes', 'ContentType')

    ct = ContentType.objects.get_for_model(Foo)

    # if any other fields need data migrated to BaseItem, do that here
    BaseItem.objects.bulk_create([
        BaseItem(
            id=obj.id,
            polymorphic_ctype=ct
        ) for obj in Foo.objects.only('id')
    ])


def populate_foo_baseitem_ptr(apps, schema_editor):
    """ Populate the `baseitem_ptr` column with the corresponding `id` value
    """
    Foo = apps.get_model('myapp', 'Foo')
    Foo.objects.update(baseitem_ptr=F('id'))


class Migration(migrations.Migration):
    """  Migration to convert Foo into a polymorphic child of BaseItem

    Notes:
    
    Since we are using UUID primary keys, we can reliably set them without
    clobbering existing records.  This approach would not be sufficient for
    auto-incrementing integer primary keys.
    
    This migration is NOT currently reversible, because when the ordered
    operations of removing a primary key before adding a new one are reversed,
    we have the removal of the *only* remaining primary key before the
    restoration of the original.
    """

    dependencies = [
        ('contenttypes', '0001_initial'),
        ('myapp', '0001_initial'),  # most recent app migration here
    ]

    operations = [
        # create a BaseItem instance for each of these Items, having the same ID
        migrations.RunPython(seed_baseitem, migrations.RunPython.noop),
        # Add the `baseitem_ptr` field (but nullable, not primary key)
        migrations.AddField(
            model_name='foo',
            name='baseitem_ptr',
            field=models.OneToOneField(parent_link=True, auto_created=True, null=True, serialize=False, to='myapp.BaseItem'),
            preserve_default=False,
        ),
        # copy `id` (and any additional relevant values) to `baseitem_ptr`
        migrations.RunPython(populate_foo_baseitem_ptr, migrations.RunPython.noop),
        # remove the original Foo primary key
        migrations.RemoveField(
            model_name='foo',
            name='id',
        ),
        # make Foo.baseitem_ptr NOTNULL, and set as primary key
        migrations.AlterField(
            model_name='foo',
            name='baseitem_ptr',
            field=models.OneToOneField(parent_link=True, auto_created=True, primary_key=True, serialize=False, to='myapp.BaseItem'),
        ),
        # run a RemoveField for any fields which are no longer needed, if data has been copied to corresponding BaseItem fields
    ]