title: Using Rails Gem Active Admin with Associations
body: <p>I'm trying out the new Rails gem <a href="http://activeadmin.info/" rel="nofollow noreferrer">http://activeadmin.info/</a> and it's working great!  However I can't find any documentation on how to use it across associations.  For example:</p>
  
  <pre><code>class Membership &lt; ActiveRecord::Base
    belongs_to :course
    belongs_to :person
  
  class Course &lt; ActiveRecord::Base
    has_many :memberships
    has_many :people,  :through =&gt; :memberships
  
  class Person &lt; ActiveRecord::Base
    has_many :memberships
    has_many :courses, :through =&gt; :memberships
  </code></pre>
  
  <p>The membership join table includes some extra data as well (ie: attendance).  I'm trying to show the membership with both the course and student name - and allow filtering / sorting on those names.  As far as I have found, Active Admin doesn't work across associations.  Has anyone else been successful in doing that, or found another gem that does?  Thanks so much!</p>