title: Django Admin filter on Foreign Key property
body: <p>I want to add a filter in an admin changelist by a property of a foreign key, e.g.</p>
  
  <pre><code>class Address(model.Models):
      street = models.CharField(max_length=25)        
      city = models.CharField(max_length=25)
      country = models.CharField(max_length=25)        
  
  class Customer(models.Model):
      name = models.CharField(max_length=25)
      address = models.ForeignKey(Address)
  </code></pre>
  
  <p>Let's say in the Customer admin changelist I want to show a filter by city and country (so show me all customers in a particular country or city).</p>
  
  <p>But the standard list_filter() functionality seems to only allow filtering by fields directly on the model and not on any of its foreign key. I've tried:</p>
  
  <pre><code>list_filter = ("address__country",)
  </code></pre>
  
  <p>or </p>
  
  <pre><code>list_filter = ("address.country",)
  </code></pre>
  
  <p>but I always get the same type of error:</p>
  
  <pre><code> 'address__country' is not a callable or an attribute 
  </code></pre>
  
  <p>Any suggestions would be welcome. Is there some special naming convention/syntax to allow filtering on FK properties?</p>