title: How can I use scope to filter many to many association in active_admin
body: <p>I just want to add scope in my index page so I can filter <code>legal_cases</code> which any <code>specific role</code> is included in their roles association, Role and LegalCase have many to many association
  I tried </p>
  
  <pre><code>ActiveAdmin.register LegalCase do
    scope :film_maker, joins(:roles).where('roles.name = ?', "Film Maker")
  end
  </code></pre>
  
  <p>but I got this error</p>
  
  <pre><code>undefined method `joins' for #&lt;ActiveAdmin::ResourceDSL:0x9ca28f4&gt;
  </code></pre>
  
  <p>any help please what should I use here instead of join?</p>