Migrating Existing Models

Existing models can be migrated to become polymorphic models. During migration, the polymorphic_ctype field needs to be populated.

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 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 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
    )


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, migrations.RunPython.noop),
    ]

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

Added in version 1.1.

When the model is created elsewhere, you can also use the reset_polymorphic_ctype() function:

from polymorphic.utils import reset_polymorphic_ctype
from myapp.models import Base, Sub1, Sub2

reset_polymorphic_ctype(Base, Sub1, Sub2)

reset_polymorphic_ctype(Base, Sub1, Sub2, ignore_existing=True)