title: how to use self-joints in activeadmin?
body: <p>I've the following Category model :</p>
  
  <pre><code>class Category &lt; ActiveRecord::Base
    belongs_to :parent, :class_name =&gt; "Category", :foreign_key =&gt; "parent_id"
    has_many :categories, :class_name =&gt; "Category", :foreign_key =&gt; "parent_id"
  end
  </code></pre>
  
  <p>and I want to be able to filter by parent category and select the parent category in add / edit categories</p>