title: Ordering with nested association
body: <p>I have an app with a nested association like this:</p>
  
  <pre><code>user has_many timesheets
  timesheet has_many invoices
  </code></pre>
  
  <p>When showing the invoice index view, I want to order the invoices on the <code>first_name</code> of the <code>user</code> it belongs to. I can do an order on the immediate parent like this:</p>
  
  <pre><code>Invoice.joins(:timesheet).order('timesheets.user_id')
  </code></pre>
  
  <p>Is it possible to go one level higher?</p>