title: How to add a report/custom page to ActiveAdmin?
body: <p>I am using <code>ActiveAdmin</code> for my Ruby on Rails application. My model consists of a couple entites such as "Plan", "Country", etc. I would like to have custom page for reporting purposes. This page should work on a collection of selected plans. So, I need <code>scopes</code> and <code>filters</code>. Currently one page associated with "Plan" model for CRUD operations. I don't want to add anything else to my data model or database since I have everything I need in my current model. How could I make such a page and access it through the menu?</p>
  
  <p>Could I associate another page with my "Plan" model as follows?</p>
  
  <pre><code>ActiveAdmin.register Plan do
    menu label: 'Report', parent: 'Planning', priority: 2
    ...
  end
  </code></pre>
  
  <p>Could I have a custom page working on set of plans selected using scopes and filters?</p>
  
  <pre><code>ActiveAdmin.register_page 'Report' do
     menu parent: 'Planning', priority: 2
     ...
  end
  </code></pre>
  
  <p>Or, how could I add a new page/route to the currently associated page with "Plan" model for reporting? Can <code>collection_action</code> help me? If yes, how could I exactly bring a collection of Plans and show them?</p>