title: Mongodb aggregate not grouping documents by date
body: <p>I have the following 3 documents in my MongoDB:</p>
  
  <pre><code>[Object_id: "tMSfNq9JR85XDaQe5"date: Sun Dec 07 2014 19:50:21 GMT+0800 (HKT)description: "Test" projectid: "S83NEGHnrefvfASrf"totalseconds: 22200__proto__: Object, 
  
  Object_id: "FeyzdMosaXCht8DKK"date: Mon Dec 15 2014 00:00:00 GMT+0800 (HKT)description: "444"projectid: "S83NEGHnrefvfASrf"totalseconds: 3660__proto__: Object, 
  
  Object_id: "cCKByxSdQMHAsRKwd"date: Mon Dec 15 2014 00:00:00 GMT+0800 (HKT)description: "555"projectid: "S83NEGHnrefvfASrf"totalseconds: 3660__proto__: Object]
  </code></pre>
  
  <p>I am trying to run the following aggregate pipeline on it to group the sum of <code>totalseconds</code> by <code>date</code> so that the end result will be something like shown below, yet each time I get the result as 3 records exactly just as input 3 documents...can someone please tell me what I might be doing wrong / missing here? Thanks</p>
  
  <pre><code>var pipeline = [
    {$group:
          {_id:{"projectId":"$projectid", "date":"$date"},
           totalHrs:{$sum:"$totalseconds"}
          }
    }
  ];
  
  
   { "Date":"Sun Dec 07 2014 19:50:21 GMT+0800 (HKT)",
     "totalseconds": "22200"
   },
   { "Date":"Sun Dec 15 2014 00:00:00 GMT+0800 (HKT)",
     "totalseconds": "7320"
   }
  </code></pre>