title: Active admin - Use controller method in Collection actions
body: <p>i want to use a custom controller action in one of the member actions of the page .  Iam getting undefined method error .</p>
  
  <pre><code>    ActiveAdmin.register_page Post do
        controller do
          def get_last_post
          end
        end
  
      collection_action :get_details, method: :get do 
        data = get_last_post
      end
  
      end
  </code></pre>
  
  <p>data = get_last_post throws error .<br>
  PS: Post is not a model . Just a page . </p>