does custom user class break applications in django?

Question

Let's say that I have subclassed User model (CustomUser) properly (as explained here: http://scottbarnham.com/blog/2008/08/21/extending-the-django-user-model-with-inheritance/)

and installed the comments app.

to access the user of a comment in the template I write:

{{comment.user}} # which provides User, not my CustomUser

and therefore,

{{comment.user.CustomProperty}} #does not work.

How can I work around it?

Answer 1

The ForeignKey from comments.Comment is to django's built-in User object, so querying comment.user will give you the parent object (ie: the base User model). However, django inheritance does provide a way to get the subclass version from the superclass:

{{ comment.user.customeruser }}

Which would then allow you to do:

{{ comment.user.customeruser.customproperty }}

I happen to think this is a weakness in Django's inheritance implementation since it doesn't exactly mirror the behaviour you'd expect from object inheritance in Python generally, but at least there is a workaround. Since I'm hardly rushing out to submit a patch with my version of the right behaviour, I can't complain :-)

---

I agree with Carl Meyer's comment: it could expensive to automatically fetch the subclass without altering the parent model's db table, and returning the instance of the subclass from the parent class query would be inconsistent with Django's promise that a queryset returns the model on which the queryset was run.

I still find in practice, however, that Django's inheritance leads to some awkward extra steps on occasion. Having used Django since 0.91, and knowing that all the different strategies for resolving object-relational mapping issues have tradeoffs, I'm very happy to have inheritance in Django now, and feel that the current implementation is excellent... so I'd hate for my original answer to be construed as a slight against the project.

As such, I thought I would edit this answer to link to an answer Carl himself provided on a solution in cases where you don't know what type the subclass is: How do I access the child classes of an object in Django without knowing the name of the child class?. He offers advice there for using the ContentType framework. Still some indirection involved, but a good, generalizable, option in the toolkit.