[{"student":1,"address":[{"a":1,"s":"df","phone":"2022452"},{"a":2,"phone":"5448421"},{"a":1,"s":"dvr"}]}]

db.collection.update({"address.phone":{$exists:true}},[{$set:{"address":{$map:{input:"$address",in:{$mergeObjects:["$$this",{phone:{$cond:[{$gt:["$$this.phone",null]},["$$this.phone"],"$$REMOVE"]}}]}}}}}])