`<orm_object>._state.adding` behaviour and model inheritance

[Ivaylo Donchev]

Let's say we have the following models:

    class User(models.Model):
        email = models.CharField(max_length=255, unique=True)
        password = models.CharField(max_length=32,  widget=forms.PasswordInput)

    class Admin(User):
        pass

    class ThirdPartyUser(User):
        pass

Steps to reproduce:

1. admin = Admin.objects.create(email='admin@admin.com')

This will create an Admin instance and User instance, with admin.user_ptr._state.adding == True

2. ThirdPartyUser.objects.create(user_ptr = admin.user_ptr)

This will raise the following error {'email': ['User with this Email address already exists.'], 'id': ['User with this ID already exists.'] }
When the admin is created, the admin.user_ptr._state.adding is True, but admin._state.adding is False (as it is already created).
If we set admin.user_ptr._state.adding = False , step 2 is passing without errors.