title: Custom search with ransacker
body: <p>I'm trying to add a custom filter to ActiveAdmin which is powered by Ransack these days. Unfortunately, <code>ransacker</code> is not documented at all and from the few resources online I fumbled together the following (in the User model):</p>
  
  <pre><code>ransacker :full_text, formatter: -&gt;(search) {
    ids = User.search_in_all_translated(search).map(&amp;:id)
    ids = ids.any? ? ids : nil
  } do |parent|
    parent.table[:id]
  end
  </code></pre>
  
  <p>The <code>search_in_all_translated</code> method returns an array of users which match the search string across all translated attributes.</p>
  
  <p>On the admin page, the following filter is defined:</p>
  
  <pre><code>filter :full_text_in,
    label: 'full text search',
    as: :string
  </code></pre>
  
  <p>The filter itself works, so filtering <code>tom</code> will list all matching records. However, the value in the filter input switches to <code>["tom"]</code>.</p>
  
  <p>Before applying the filter:</p>
  
  <p><img src="https://i.stack.imgur.com/dHUOK.png" alt="enter image description here"></p>
  
  <p>After applying the filter:</p>
  
  <p><img src="https://i.stack.imgur.com/O53uY.png" alt="enter image description here"></p>
  
  <p>Any ideas how to fix this?</p>