A django application that provides a simple way to retrieve models type casted
to their original ContentType.
>>> pip install django-polymodelsMake 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 __str__(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)
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.
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.
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.
- Inspired by a post of Jeff Elmores.
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