title: Ransack search through scope
body: <p>I have a model <code>Task</code> with a scope named <code>done</code>.
  A <code>Task</code> belongs to a <code>Project</code> and to a <code>User</code>.</p>
  
  <p>I need to filter users by projects in which they have a done <code>task</code>.</p>
  
  <pre><code>simple_form_for @q do |form|
    form.input :tasks_done_project_id, collection: Project.ordered
  </code></pre>
  
  <p>But using "_done" doesn't work.</p>
  
  <p>I thought about a ransacker:</p>
  
  <pre><code>  ransacker :done do |parent|
      parent.done.table[:id]
    end
  </code></pre>
  
  <p>But doesn't work neither. Any Idea?</p>