title: Django admin filtering users
body: <p>I have an app which is like a <code>store</code> where each store has many <code>products</code> and a product has many <code>orders</code> (<code>product_id</code>, <code>user_id</code>). On the products page I show the number of users that own that product and I want to place a link that will redirect me to the list for those users.</p>
  
  <p>I create the link as follows:</p>
  
  <pre><code>def users_link( self, product ):
      url = urlresolvers.reverse( 'admin:auth_user_changelist' )
      return u'&lt;a href="%s?orders__product_id=%d"&gt;%s&lt;/a&gt;' %( url, product.pk, product.users_count )
  </code></pre>
  
  <p>But I get the following error:</p>
  
  <pre><code>DisallowedModelAdminLookup at /admin/auth/user/
  Filtering by orders__product_id__exact not allowed
  </code></pre>
  
  <p>Even if I add <code>orders__product_id</code> to the list of allowed <code>list_filter</code> for users it still denies me. I can see the filter on the users page but when I click on it it just errors out. How can I achieve this?</p>