django-polymodels 1.3.0

Installation

>>> pip install polymodels

Make sure 'django.contrib.contenttypes' and 'polymodels' are in your INSTALLED_APPS

INSTALLED_APPS += ('django.contrib.contenttypes', 'polymodels')

Usage

Subclass PolymorphicModel, an abstract model class.

from django.db import models
from polymodels.models import PolymorphicModel

class Animal(PolymorphicModel):
    name = models.CharField(max_length=255)

    def __unicode__(self):
        return self.name

class Mammal(Animal):
    pass

class Dog(Mammal):
    pass

class Reptile(Animal):
    pass

class Snake(Reptile):
    class Meta:
        proxy = True

Objects are created the same way as usual and their associated ContentType is saved automatically:

>>> animal = Animal.objects.create(name='animal')
>>> mammal = Mammal.objects.create(name='mammal')
>>> reptile = Reptile.objects.create(name='reptile')
>>> snake = Snake.objects.create(name='snake')

To retreive type casted instances from the Animal.objects manager you just have to use the select_subclasses method.

>>> Animal.objects.select_subclasses()
[<Animal: animal>, <Mammal: mammal>, <Reptile: reptile>, <Snake: snake>]

You can also retreive a subset of the subclasses by passing them as arguments to select_subclass.

>>> Animal.objects.select_subclasses(Reptile)
[<Reptile: reptile>, <Snake: snake>]

Or directly from subclasses managers.

>>> Reptile.objects.select_subclasses(Snake)
[<Snake: snake>]

Note that you can also retrieve original results by avoiding the select_subclasses call.

>>> Animal.objects.all()
[<Animal: animal>, <Animal: mammal>, <Animal: reptile>, <Animal: snake>]

It’s also possible to select only instances of the model to which the manager is attached by using the exclude_subclasses method.

>>> Mammal.objects.all()
[<Mammal: mammal>]

Each instance of PolymorphicModel has a type_cast method that knows how to convert itself to the correct ContentType.

>>> animal_snake = Animal.objects.get(pk=snake.pk)
<Animal: snake>
>>> animal_snake.type_cast()
<Snake: snake>
>>> animal_snake.type_cast(Reptile)
<Reptile: snake>

If the PolymorphicModel.content_type fields conflicts with one of your existing fields you just have to subclass polymodels.models.BasePolymorphicModel and specify which field polymodels should use instead by defining a CONTENT_TYPE_FIELD attribute on your model. This field must be a ForeignKey to ContentType.

from django.contrib.contenttypes.models import ContentType
from django.db import models
from polymodels.models import BasePolymorphicModel

class MyModel(BasePolymorphicModel):
    CONTENT_TYPE_FIELD = 'polymorphic_ct'
    polymorphic_ct = models.ForeignKey(ContentType)

How it works

Under the hood select_subclasses calls seleted_related to avoid unnecessary queries and filter if you pass some classes to it. On queryset iteration, the fetched instanced are converted to their correct type by calling BasePolymorphicModel.type_cast. Note that those lookups are cached on class creation to avoid computing them on every single query.

Caution

Prior to Django 1.6 it was not possible to issue a select_related over multiple one-to-one relationships. For example, given the models defined above, Animal.objects.select_related('mammal__dog') would throw a strange TypeError. To work around this issue, select_subclasses limits such lookups to one level deep if you’re using a version of django with this issue.

Note of the author

I’m aware there’s already plenty of existing projects tackling the whole model-inheritance-type-casting-thing such as django-polymorphic. However I wanted to implement this feature in a lightweight way: no __metaclass__ or __init__ overrides while using django’s public API as much as possible. In the end, this was really just an extraction of django-mutant’s own mecanism of handling this since I needed it as a standalone app for another project.

Contribute

If you happen to encounter a bug or would like to suggest a feature addition please file an issue or create a pull request containing tests.

Credits

• Inspired by a post of Jeff Elmores.