[{_id:ObjectId("56edbb4d5f084a51131dd4c6"),userRef:ObjectId("56edbb4d5f084a51131dd4c6"),serialNumber:"A123123"}]

db.collection.aggregate([{"$addFields":{"userRef":{"$toString":"$userRef"}}},{"$group":{"_id":{"$concat":["$userRef","-","$serialNumber"]}}}])