[{"_id":ObjectId("60ddc26b03edfb7a6b424f10"),"member":[{"joinDate":"2021-07-01 12:00:00.000","userName":"John"},{"joinDate":"2021-07-01 13:00:00.000","userName":"Maria"}],"createdAt":"2021-07-01 11:00","roomName":"roomA"},{"_id":ObjectId("60ddbcccd495870939756dcb"),"member":[{"joinDate":"2021-07-01 11:00:00.000","userName":"George"},{"joinDate":"2021-07-01 14:00:00.000","userName":"Jack"}],"createdAt":"2021-07-01 10:00","roomName":"roomB"},{"_id":ObjectId("60ddbcccd495870939756dcc"),"createdAt":"2021-07-01 12:00","roomName":"roomC"}]

db.collection.aggregate([{$match:{member:{$exists:true}}},{$sort:{"member.joinDate":-1}},{$addFields:{member:{$arrayElemAt:["$member",{$indexOfArray:["$member.joinDate",{$max:"$member.joinDate"}]}]}}},{$project:{createdAt:1,roomName:1,latestJoinDate:"$member.joinDate",latestUserName:"$member.userName"}}])