[{"_id":"12345","to":"fnk34","messages":[{id:"1",title:"hello",body:"body",createdAt:123},{id:"2",title:"hello",body:"body",createdAt:3},{id:"3",title:"hello",body:"body",createdAt:13},]},]

db.collection.update({to:"fnk34"},{$push:{messages:{"$each":[{id:"4",title:"hello",body:"body",createdAt:1234}],"$sort":{"createdAt":-1}}}},{upsert:true})