Database
[{_id:ObjectId("609977b0e8e1c615cb551bf5"),activityId:"123456789",updateDateTime:"2021-03-24T20:12:02Z"},{_id:ObjectId("739177b0e8e1c615cb551bf5"),activityId:"123456789",updateDateTime:"2021-03-24T20:15:02Z"},{_id:ObjectId("805577b0e8e1c615cb551bf5"),activityId:"123456789",updateDateTime:"2021-03-24T20:18:02Z"}]
Query
db.collection.aggregate([{$sort:{updateDateTime:-1}},{$group:{_id:"$activityId",record:{$first:"$$ROOT"}}},{$replaceRoot:{newRoot:"$record"}}])