django: how to extend an existing model

there are plenty of examples the tell you how to extend the user model BUT I cannot find a real, complete and documented example on how to extend an existing model without having to follow the "user profile pattern" (and honestly I wonder why).

In short, my use case is the following: I need开发者_开发技巧 to extend django-lfs's product model.

In LFS is registered like this (in lfs.catalog.admin):

from django.contrib import admin

[...]
from lfs.catalog.models import Product
[...]
class ProductAdmin(admin.ModelAdmin):
    prepopulated_fields = {"slug": ("name", )}
admin.site.register(Product, ProductAdmin)
[...]

I tried to register mine (that subclasses it) but I got:

django/contrib/admin/sites.py", line 78, in register raise AlreadyRegistered('The model %s is already registered' % model.name)

So, someone suggested me that I have to unregister that object and register mine.

I did it like this:

from lfs.catalog.models import Product
from lfs.catalog.admin import ProductAdmin

admin.site.unregister(Product)

from lfs_product_highlights.catalog.models import Product

admin.site.register(Product,ProductAdmin)

No errors this time BUT there's no change, my custom fields are nowhere to be seen.

Any hints?

---

The reason why it's difficult is because of the object-relational impedance mismatch (love that phrase). Objects and classes do not map perfectly onto relational databases: ORMs like Django's attempt to smooth out the edges, but there are some places where the differences are just too great. Inheritance is one of these: there is simply no way to make one table "inherit" from another, so it has to be simulated via Foreign Keys or the like.

Anyway, for your actual problem, I can't really see what's going on but one possible way to fix it would be to subclass ProductAdmin as well, and specifically set the model attribute to your subclassed model.