Right way to return proxy model instance from a base model instance in Django?

user_85271 0
Viewed 9381

Say I have models:

class Animal(models.Model):
    type = models.CharField(max_length=255)

class Dog(Animal):
    def make_sound(self):
        print "Woof!"
    class Meta:
        proxy = True

class Cat(Animal):
    def make_sound(self):
        print "Meow!"
    class Meta:
        proxy = True

Let's say I want to do:

 animals = Animal.objects.all()
 for animal in animals:
     animal.make_sound()

I want to get back a series of Woofs and Meows. Clearly, I could just define a make_sound in the original model that forks based on animal_type, but then every time I add a new animal type (imagine they're in different apps), I'd have to go in and edit that make_sound function. I'd rather just define proxy models and have them define the behavior themselves. From what I can tell, there's no way of returning mixed Cat or Dog instances, but I figured maybe I could define a "get_proxy_model" method on the main class that returns a cat or a dog model.

Surely you could do this, and pass something like the primary key and then just do Cat.objects.get(pk = passed_in_primary_key). But that'd mean doing an extra query for data you already have which seems redundant. Is there any way to turn an animal into a cat or a dog instance in an efficient way? What's the right way to do what I want to achieve?

proxy-classes django-models django python
5 Answers

I played around with a lot of ways to do this. In the end the most simple seems to be the way forward. Override __init__ of the base class.

class Animal(models.Model):
    type = models.CharField(max_length=255)
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.__class__ = eval(self.type)

I know eval can be dangerous, bla bla bla, but you can always add safeguarding/validation on the type choice to ensure it's what you want to see. Besdies that, I can't think of any obvious pitfalls but if i find any i'll mention them/ delete the answer! (yeah i know the question is super old, but hopefully this'll help others with the same problem)

user_26106130

You can perhaps make Django models polymorphic using the approach described here. That code is in early stages of development, I believe, but worth investigating.

user_1189030
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