Using django, how do I create an instance of a proxy object from an instance of a superclass object?

Question

I'm still a little confused by the relationship of proxy models to their superclasses in django. Now I have a question, how to get an instance of the Proxy model from an already received instance of the superclass?

So let's say I have:

class Animal(models.Model):
   type = models.CharField(max_length=20)
   name = models.CharField(max_length=40)

class Dog(Animal):  
   class Meta:
       proxy = True

   def make_noise(self):  
       print "Woof Woof"  

Class Cat(Animal):  
   class Meta:
       proxy = True

   def make_noise(self):  
       print "Meow Meow"

animals = Animal.objects.all()
for animal in animals:
   if (animal.type == "cat"):
      animal_proxy = # make me a cat
   elif (animal.type == "dog"):
      animal_proxy = # make me a dog
   animal_proxy.make_noise()

OK So .. What is included in "# make me a cat", which does not require a query back to the database, for example:

animal_proxy = Cat.objects.get(id=animal.id)

Is there an easy way to create a Cat instance from an Animal instance, which I know is a cat?

Answer 1

. type . , , :

for animal in animals:
   if (animal.type == "cat"): 
      animal_proxy = # make me a cat

Django. ( ).

, :

"" -. , Dog.objects Animal type="dog" Cat.objects Animal type="cat".

class TypeAwareManager(models.Manager):
    def __init__(self, type, *args, **kwargs):
        super(TypeAwareManager, self).__init__(*args, **kwargs)
        self.type = type

    def get_query_set(self):
        return super(TypeAwareManager, self).get_query_set().filter(
              type = self.type)

class Dog(Animal):
    objects = TypeAwareManager('dog')
    ...

class Cat(Animal):
    objects = TypeAwareManager('cat')
    ...

-, . . itertools.chain Querysets.

from itertools import chain
q1 = Cat.objects.all() # [<Cat: Daisy [cat]>]

q2 = Dog.objects.all() # [<Dog: Bruno [dog]>]

for each in chain(q1, q2): 
    each.make_noise() 

# Meow Meow
# Woof Woof

Answer 2

:

def reklass_model(model_instance, model_subklass):

    fields = model_instance._meta.get_all_field_names()
    kwargs = {}
    for field_name in fields:
        try:
           kwargs[field_name] = getattr(model_instance, field_name)
        except ValueError as e: 
           #needed for ManyToManyField for not already saved instances
           pass

    return model_subklass(**kwargs)

animals = Animal.objects.all()
for animal in animals:
   if (animal.type == "cat"):
      animal_proxy = reklass_model(animal, Cat)
   elif (animal.type == "dog"):
      animal_proxy = reklass_model(animal, Cat)
   animal_proxy.make_noise()

# Meow Meow
# Woof Woof

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