title: Mongodb aggregate for nested subdocument field
body: <p>I'm trying to get the sum of "Duration" from mongo subdocument's using aggregate,
  Here I have <code>event_id = "5656b3655b9e5c8812000007",</code> I need to get the sum of "Duration" field by matching the event_id = "5656b3655b9e5c8812000007" in above example.</p>
  
  <p><strong>Example:</strong>  </p>
  
  <pre><code>"history" : [
          {
              "timestamp" : "2015-10-26 14:39:15", 
              "event" : "Creation", 
              "createdby" : "Ws", 
              "data" : [
                  {
                      "crm_base_contact_id" : "1847", 
                      "crm_imported_files_id" : ""
                  }
              ]
          }, 
          {
              "timestamp" : "2015-10-26 15:12:28", 
              "event" : "Task", 
              "createdby" : "Auto-Filter", 
              "data" : [
                  {
                      "Campaign ID" : "219", 
                      "Campaign Name" : "ASA", 
                      "Task ID" : "2541639", 
                      "Filter Name" : "ASA", 
                      "Filter ID" : "826"
                  }
              ]
          }, 
          {
              "event_id" : ObjectId("5656b3655b9e5c8812000007"), 
              "timestamp" : "2015-11-26 08:23:17", 
              "event" : "Session", 
              "createdby" : "ABC", 
              "data" : [
                  {
                      "Campaign ID" : "219", 
                      "Task ID" : "2541639", 
                      "viopCalls" : [
                          {
                              "timestamp" : "2015-11-26 08:25:42", 
                              "CallID" : "46", 
                              "Duration" : NumberInt(5), 
                              "TelNumber" : "0685356189"
                          }, 
                          {
                              "timestamp" : "2015-11-26 08:26:13", 
                              "CallID" : "45", 
                              "Duration" : NumberInt(3), 
                              "TelNumber" : "0685356189"
                          }
                      ]
                  }
              ]             
          }
      ], 
  
  
  Expected output : Duration : 8
  </code></pre>
  
  <p>Please can anyone help me to get the sum of duration column !!!</p>