[{"username":"Dhruv70","notes":[{"topic":"hello","content":"world"},{"topic":"bye","content":"universe"},{"topic":"xxxxxxx","content":"xx"}],"__v":31}]

db.collection.update({"username":"Dhruv70"},[{"$set":{"notes":{$concatArrays:[{$ifNull:["$notes",[]]},[{"topic":"new note","content":"to insert"}]]}}}],{upsert:true})