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

[guettli]

Hi,

this ticket is seven years old

https://code.djangoproject.com/ticket/7623

{{{

As it exists now, multi-table inheritance does not allow for the creation of a child model instance that inherits from an existing parent model instance. For example:

Parent Class-

class Place(models.Model):
    name = models.CharField(max_length=50)
    address = models.TextField(max_length=150)

Child Classes-

class Restaurant(Place):
    place = models.OneToOneField(Place, parent_link=True)
    cuisine = models.CharField(max_length=75)
    rating = models.IntegerField()

class Bar(Place):
    parent = models.OneToOneField(Place, parent_link=True)
    happy_hour = models.BooleanField()
    beers_on_tap = models.ManyToManyField("Beers", null=True, blank=True)

Sample Use-case-

When the system is first deployed, a restaurant instance is created. Later, the restaurant adds a bar to increase revenue, and we now want to create a Bar model instance for the parent Place for the restaurant. I would propose the following interface for doing so:

parentPlace = Restaurant.objects.get(name__iexact="Bob's Place").parent
barInstance = Bar(parent=parentPlace, happy_hour=True)

However, if you attempt to create an instance in this manner now, you receive a DatabaseIntegrityError, saying that a Place object with that id already exists.

}}}

How to get this solved?

There are work arounds:

http://stackoverflow.com/questions/4064808/django-model-inheritance-create-sub-instance-of-existing-instance-downcast

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

Or is there a better solution in current django versions?

 

[Mario Gudelj]

Have you had a look at http://django-model-utils.readthedocs.org/en/latest/managers.html#inheritancemanager? I'm not sure if it will help but it's kind of related to what you've posted.

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[guettli]

Thank you for this link. The InheritanceManager is good to know about. But it does not help creating the child_instance from a base instance. At least the docs don't tell about it. Or I was blind.

[Simon Charette]

Hi Guettli,

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.

Simon

[guettli]

Thank you Simon. I did not know save_base(raw=True) before.