title: Multi-level sum in mongoDb
body: <p>I am working on mongoDB with Laravel using <a href="https://github.com/jenssegers/laravel-mongodb" rel="nofollow">jenssegers</a>. And i am trying to sum based on time.estimated_time_in_number.</p>
  
  <p>I have MongoDB document which contains following data.</p>
  
  <pre><code>"time": [
       {
         "id": "QZMV154213",
         "estimated_time_in_number": 4.41
      },
      {
         "id": "QZMV154213",
         "estimated_time_in_number": 2.25
      },{
         "id": "QZMV154213",
         "estimated_time_in_number": 5
      }
    ]
  </code></pre>
  
  <p>I am using following query and it gives me 0 rather then 11.66.</p>
  
  <pre><code>Task::sum('time.estimated_time_in_number');
  </code></pre>