{"_id":"8NH5rNCz47fyoo3z2",
"Cpe":{"Srno":"GZDBDt2NmpMv54j","date":{"$date":"2016-12-04T14:51:26.452Z"}},
"serialNumber":4.703961341e+09,
"Device":
{"object_id":1,

    "terminal_id":3,"Info":{"FirmwareVersion":{"value":"5.9.2"},
    "Model":{"value":"lunus    Rustic HRK   paxton -  989"},
    "DeviceSerialNumber":{"value":3.830919496e+09},
    "BatteryType":{"value":"Handcrafted  lithium  battery"},
    "SavedPriority":{"value":7}}}
}

db.AllDevices.aggregate([{$sort:{"Cpe.date":-1}},{$group:{_id:"$Cpe.Srno","latest":{$first:"$Cpe.date"}}},])

{ "_id" : "qst3pnS3EQi8uHb", "latest" : ISODate("2016-12-04T14:51:26.487Z") }
{ "_id" : "Ur45SS1I3Yaji2p", "latest" : ISODate("2016-12-04T14:51:26.513Z") }
{ "_id" : "9ZZXVVEAQ5pA9Ax", "latest" : ISODate("2016-12-04T14:51:26.518Z") }

db.AllDevices.aggregate([{$sort:{"Cpe.date":-1}},{$group:{_id:"$Cpe.Srno","latest":{$first:"$Cpe.date"}}},{$project:{Srno:"$_id",datetag:"$latest",_id:0}},{$match:{"Cpe.Srno":"$Srno","Cpe.date":"$datetag"}}])