Django model inheritance: create sub-instance of existing instance (downcast)?

I'm trying to integrate a 3rd party Django app that made the unfortunate decision to inherit from django.contrib.auth.models.User, which is a big no-no for pluggable apps. Quoting Malcolm Tredinnick:

> More importantly, though, just as in Python you cannot "downcast" with Django's model inheritance. That is, if you've already created the User instance, you cannot, without poking about under the covers, make that instance correspond to a subclass instance that you haven't created yet.

Well, I'm in the situation where I need to integrate this 3rd party app with my existing user instances. So, if hypothetically I am indeed willing to poke about under the covers, what are my options? I know that this doesn't work:

extended_user = ExtendedUser(user_ptr_id=auth_user.pk)
extended_user.save()

There's no exception, but it breaks all kinds of stuff, starting with overwriting all the columns from django.contrib.auth.models.User with empty strings...

This should work:

extended_user = ExtendedUser(user_ptr_id=auth_user.pk)
extended_user.__dict__.update(auth_user.__dict__)
extended_user.save()

Here you're basically just copying over the values from the auth_user version into the extended_user one, and re-saving it. Not very elegant, but it works.

I found this answer by asking on django-user mailing list:

https://groups.google.com/d/msg/django-users/02t83cuEbeg/JnPkriW-omQJ

This isn't part of the public API but you could rely on how Django loads fixture internally.

parent = Restaurant.objects.get(name__iexact="Bob's Place").parent
bar = Bar(parent=parent, happy_hour=True)
bar.save_base(raw=True)

Keep in mind that this could break with any new version of Django.

If you don't like __dict__.update solution you can do this:

for field in parent_obj._meta.fields
    setattr(child_obj, field.attname, getattr(parent_obj, field.attname))

I am using Django 1.6, and my ExtendedUser model is from OSQA (forum.models.user.User). For some bizarre reason the above solutions with dict.__update__ and with setattr sometimes fail. This may have to do with some other models that I have, that are putting constrains on the user tables. Here are two more workarounds that you can try:

Workaround #1:

extended_user = ExtendedUser(user_ptr_id = user.pk)
extended_user.save() # save first time
extended_user.__dict__.update(user.__dict__)
extended_user.save() # save second time

Workaround #2:

extended_user = ExtendedUser(user_ptr_id = user.pk)
extended_user.__dict__.update(user.__dict__)
extended_user.id=None
extended_user.save()

That is, sometimes saving the new child instance fails if you set both pk and id, but you can set just pk, save it, and then everything seems to work fine.

There is an open bug for this very question: https://code.djangoproject.com/ticket/7623

The proposed patch (https://github.com/django/django/compare/master...ar45:child_object_from_parent_model) is not using obj.__dict__ but creates an dictionary with all field values cycling over all fields. Here a simplified function:

def create_child_from_parent_model(child_cls, parent_obj, init_values: dict):
    attrs = {}
    for field in parent_obj._meta._get_fields(reverse=False, include_parents=True):
        if field.attname not in attrs:
            attrs[field.attname] = getattr(parent_obj, field.attname)
    attrs[child_cls._meta.parents[parent_obj.__class__].name] = parent_obj
    attrs.update(init_values)
    print(attrs)
    return child_cls(**attrs)

create_child_from_parent_model(ExtendedUser, auth_user, {})

This method has the advantage that methods that are overwritten by the child are not replaced by the original parent methods. For me using the original answers obj.__dict__.update() led to exceptions as I was using the FieldTracker from model_utils in the parent class.

What about something like this:

from django.forms.models import model_to_dict

auth_user_dict = model_to_dict(auth_user)
extended_user = ExtendedUser.objects.create(user_ptr=auth_user, **auth_user_dict)

@guetti's answer worked for me with little update => The key was parent_ptr

parent_object = parent_model.objects.get(pk=parent_id)  
new_child_object_with_existing_parent = Child(parent_ptr=parent, child_filed1='Nothing')
new_child_object_with_existing_parent.save()

I wanted to create entry in my profile model for existing user, my model was like

from django.contrib.auth.models import User as user_model
class Profile(user_model):
     bio = models.CharField(maxlength=1000)
     another_filed = models.CharField(maxlength=1000, null=True, blank=True)

At some place I needed to create profile if not exists for existing user so I did it like following,

The example that worked for me

from meetings.user import Profile
from django.contrib.auth.models import User as user_model

user_object = user_model.objects.get(pk=3)  
profile_object = Profile(user_ptr=user_object, bio='some')
profile_object.save()