[{"items":[{_id:0,name:"john"}]},]

db.collection.update({},[{"$set":{"items":{$concatArrays:[{$ifNull:["$items",[]]},[{$mergeObjects:[{name:"Bonny"},{_id:{$size:{$ifNull:["$items",[]]}}}]}]]}}}],{"multi":false,"upsert":false})