title: aggregation with $group stage and $sum operator
body: <p>I want to sum all "total" fields values using aggregation and group stage.
  my code and documents is here</p>
  
  <p><strong>documents</strong></p>
  
  <pre><code>{
      "_id": "57a068477b2l51ec16eb7das",
      "userID":"5742c6eedsaadsd93573e",
      "profileID":"5742aee49adv520593573c",
      "date": 1470130247779,
      "updateDate": 1470130361342,
      "total": 2
  }
  {
      "_id": "57a068477b2l51ec16eb983",
      "userID":"5742c6eedsaadsd93573e",
      "profileID":"5742aee49adv520593573c",
      "date": 1470130247779,
      "updateDate": 1470130361342,
      "total": 1
  }
  </code></pre>
  
  <p>I'm doing below query for fetch sum of "total" values</p>
  
  <pre><code>    RecallProfile.aggregate([
          {
              "$match": {
                  "updateDate": {
                      "$gte": from, "$lte": to
                  }
              }
          },
          {
              $group: {
                  "_id": "$updateDate",
                  "totalRecord": {"$sum": "$total"}
              }
          }
      ], function (err, result) {
          if (err) {
              console.log('error ', err);
              res.sendStatus(500);
          } else {
              console.log("result", result);
              res.sendStatus(200);
          }
      })
  </code></pre>
  
  <p>Its giving "result []" log.</p>