title: Rails - ActiveAdmin - Filter another model
body: <p>I use rails 4.2.4, and activeadmin gem.</p>
  
  <p>I have 2 models.</p>
  
  <p><code>User model: email:string, password: string</code>.</p>
  
  <p>User model <code>has_one</code> User Detail.</p>
  
  <p>UserDetail is <code>belongs_to</code> User, it have <code>name:string</code>.</p>
  
  <p>But, I filter name in</p>
  
  <pre class="lang-ruby prettyprint-override"><code>ActiveAdmin.register User do
    filter :name
  end
  </code></pre>
  
  <p>I don't see name field.</p>
  
  <p>Thanks.</p>