Migrating existing models to polymorphic

Existing models can be migrated to become polymorphic models. During the migrating, the polymorphic_ctype field needs to be filled in.

This can be done in the following steps:

1. Inherit your model from PolymorphicModel.
2. Create a Django migration file to create the polymorphic_ctype_id database column.
3. Make sure the proper ContentType value is filled in.

Filling the content type value

The following Python code can be used to fill the value of a model:

from django.contrib.contenttypes.models import ContentType
from myapp.models import MyModel

new_ct = ContentType.objects.get_for_model(MyModel)
MyModel.objects.filter(polymorphic_ctype__isnull=True).update(polymorphic_ctype=new_ct)

The creation and update of the polymorphic_ctype_id column can be included in a single Django migration. For example:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals
from django.db import migrations, models


def forwards_func(apps, schema_editor):
    MyModel = apps.get_model('myapp', 'MyModel')
    ContentType = apps.get_model('contenttypes', 'ContentType')

    new_ct = ContentType.objects.get_for_model(MyModel)
    MyModel.objects.filter(polymorphic_ctype__isnull=True).update(polymorphic_ctype=new_ct)


def backwards_func(apps, schema_editor):
    pass


class Migration(migrations.Migration):

    dependencies = [
        ('contenttypes', '0001_initial'),
        ('myapp', '0001_initial'),
    ]

    operations = [
        migrations.AddField(
            model_name='mymodel',
            name='polymorphic_ctype',
            field=models.ForeignKey(related_name='polymorphic_myapp.mymodel_set+', editable=False, to='contenttypes.ContentType', null=True),
        ),
        migrations.RunPython(forwards_func, backwards_func),
    ]

It’s recommended to let makemigrations create the migration file, and include the RunPython manually before running the migration.