Django model inheritance problem. How to solve?

Question

I have an existing app with the following model

class Contact(models.Model):
     lastname = models.CharField(max_length=200)
     firstname = models.CharField(max_length=200)
     ...

class Journalist(Contact):
     pass

I have a Contact in my database and I would like that it becomes a Journalist .

In raw sql, it seems as simple as insert into app_journalist values (25624); . In this example 25624 is the id of the existing contact. It seems to work ok and the django app seems happy.

However, I would like to make the same thing with the django ORM. I have tried several thinks like forcing the journalist id ( Journalist(id=25624) ) but it creates a new contact rather than linking to the existing one.

Is it possible do to that with the Django ORM? How?

Thanks in advance for your help

Answer 1

One way to solve this is (without changing your model structure) to set the contact_ptr attribute of the Journalist instance to the appropriate Contact instance. For eg

contact = Contact.objects.get(pk = 25624)
journalist = Journalist(contact_ptr = contact)
journalist.save()

This becomes easier to understand if you first look at the table app_journalist . It has but one column, contact_ptr_id . So when you execute insert into app_journalist values (25624) you are setting contact_ptr_id = 25624 at the SQL level. Correspondingly you should set the contact_ptr = <instance of Contact> at the ORM level.

Update

There are other ways to solve the problem but they would require changes to your existing models. As @bugspy.net pointed out you can use a generic relationship. Alternately you can declare an additional type field to specify whether the contact is a journalist, a colleague etc.

Update 2

Also take a look at this demo snippet (and the complete code ) that lets you use polymorphic inheritance (SQLAlchemy already does this).

Update 3

As @luc himself pointed out (see comment below)

journalist = Journalist(contact_ptr = contact)

alone will not suffice. This will overwrite the firstname and lastname of the contact to "" . To avoid this you have to explicitly assign each field to Journalist .

Answer 2

Django's Contenttypes framework is really handy. You could use it to represent different contact types:

from django.db import models
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes import generic

class Contact(models.Model):
     lastname = models.CharField(max_length=200)
     firstname = models.CharField(max_length=200)
     content_object = generic.GenericForeignKey('content_type', 'object_id')

Answer 3

Inheritance at the model level is usually not such a good idea. Most ORM let you do it. Most ORM are even proud to be able to do it. At the model level, the famous " prefer composition over inheritance " is more true than ever.

In your case, instead of saying : "A journalist IS A person" , you could instead say : "A person has a job, which in this case is journalist" . This would be represented by a Person class composed with a Job class. One of the job could be "journalist".

The composition approach lets you have a person change job, or even have multiple jobs.

Of course, this doesnt directly answer your question, but the other answers are already pretty good !