Mongodb aggregate for nested subdocument field - mongodb

Question

I'm trying to get the sum of "Duration" from mongo subdocument's using aggregate,
Here I have event_id = "5656b3655b9e5c8812000007", I need to get the sum of "Duration" field by matching the event_id = "5656b3655b9e5c8812000007" in above example.
Example:
"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
Please can anyone help me to get the sum of duration column !!!

Answer 1

Use can use the following aggregate query:
db.collection.aggregate(
{$unwind: '$history'},
{$match: {'history.event_id': ObjectId("5656b3655b9e5c8812000007")}},
{$unwind: '$history.data'},
{$unwind: '$history.data.viopCalls'},
{$group: {_id: null, duration: {$sum: '$history.data.viopCalls.Duration'}}}
)
Result:
{ "_id" : null, "duration" : 8 }