title: Multiple groupBy on Laravel Collection
body: <p>I am trying to group a collection by two clauses in Laravel 5, but it only groups it by the first one. Can someone please help me out? Here's my code.</p>
  
  <pre><code>$query = Activity::all()-&gt;groupBy('performed_at_year_month', 'performer_id')-&gt;toJson();
  </code></pre>