title: django: how to extend an existing model
body: <p>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).</p>
  
  <p>In short, my use case is the following: I need to extend django-lfs's product model.</p>
  
  <p>In LFS is registered like this (in lfs.catalog.admin):</p>
  
  <pre><code>from django.contrib import admin
  
  [...]
  from lfs.catalog.models import Product
  [...]
  class ProductAdmin(admin.ModelAdmin):
      prepopulated_fields = {"slug": ("name", )}
  admin.site.register(Product, ProductAdmin)
  [...]
  </code></pre>
  
  <p>I tried to register mine (that subclasses it) but I got:</p>
  
  <p>django/contrib/admin/sites.py",
  line 78, in register
      raise AlreadyRegistered('The model %s is already registered' %
  model.<strong>name</strong>)</p>
  
  <p>So, someone suggested me that I have to unregister that object and register mine.</p>
  
  <p>I did it like this:</p>
  
  <pre><code>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)
  </code></pre>
  
  <p>No errors this time BUT there's no change, my custom fields are nowhere to be seen.</p>
  
  <p>Any hints?</p>