Polymorphic models implementation for django
A django application that provides a simple way to retrieve models type casted to their original ContentType.
>>> pip install django-polymodels
Make sure 'django.contrib.contenttypes' and 'polymodels' are in your INSTALLED_APPS
INSTALLED_APPS += ('django.contrib.contenttypes', 'polymodels')
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.
- Inspired by a post of Jeff Elmores.
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