title: Rails 3 ActiveAdmin. How to use ONE field for SIX columns on a search in Active Admin?
body: <p>How can I add a custom filter or create a custom search field with ActiveAdmin?</p>
  
  <p>I need to generate a report on wire transfers. There are these fields in the Invoices model</p>
  
  <ul>
  <li>wire_transfer_1_ar</li>
  <li>wire_transfer_2_ar</li>
  <li>wire_transfer_3_ar</li>
  <li>wire_transfer_1_customer</li>
  <li>wire_transfer_2_customer</li>
  <li>wire_transfer_3_customer</li>
  </ul>
  
  <p>So I have this</p>
  
  <pre><code>filter :wire_transfer_1_ar
  filter :wire_transfer_2_ar
  filter :wire_transfer_3_ar
  filter :wire_transfer_1_customer
  filter :wire_transfer_2_customer
  filter :wire_transfer_2_customer
  </code></pre>
  
  <p>So when a wire transfer is found, then I can click a link to generate a report.</p>
  
  <p>How can I use only ONE field, to search for all those 6 fields?</p>