Django access to subclasses items from abstract class

Question

class Animal(models.Model):
    ....
    class Meta:
        abstract = True

class Cat(models.Model, Animal):
    ...

class Dog(models.Model, Animal):
    ....
         


        

Answer 1

You might be interested in django_polymorphic

Example from project docs:

When we store models that inherit from a Project model...

>>> Project.objects.create(topic="Department Party")
>>> ArtProject.objects.create(topic="Painting with Tim", artist="T. Turner")
>>> ResearchProject.objects.create(topic="Swallow Aerodynamics", supervisor="Dr. Winter")

...and want to retrieve all our projects, the subclassed models are returned:

>>> Project.objects.all()
[ <Project:         id 1, topic "Department Party">,
  <ArtProject:      id 2, topic "Painting with Tim", artist "T. Turner">,
  <ResearchProject: id 3, topic "Swallow Aerodynamics", supervisor "Dr. Winter"> ]

Answer 2

This is not possible in one query. You have two options, one use to use django-model-utils or you can use django_polymorphic.

Polymorphic is better suited to your task, however django-model-utils is made by a very prominent member of the django community and as such has a lot of good support.

If I had to choose, I'd choose django-model-utils since its made by a member of the django team, and thus will be supported. Polymorphic is supported by divio, which is a private company that heavily uses django based in Switzerland.

As for how to select Sub-classes. You need to do two things using django-model-utils. Firstly, you need to change the objects variable in your model to InheritanceManager() like so (adapted from docs):

from model_utils.managers import InheritanceManager

class Place(models.Model):
    # ...
    objects = InheritanceManager()

class Restaurant(Place):
    # ...

class Bar(Place):
    # ...

nearby_places = Place.objects.filter(location='here').select_subclasses()
for place in nearby_places:
    # "place" will automatically be an instance of Place, Restaurant, or Bar

The code above will return all Bars and Restaurants because it uses the select_subclasses.