Database
[{_id:"1",user_id:"userA",date:"2022-10-20 11:00:00.000000000"},{_id:"2",user_id:"userA",date:"2022-10-25 18:00:00.000000000"},{_id:"3",user_id:"userB",date:"2022-10-25 18:00:00.000000000"},{_id:"4",user_id:"userC",date:"2022-10-25 18:00:00.000000000"}]
Query
db.collection.aggregate([{$group:{_id:"$user_id",root:{"$bottom":{"sortBy":{"date":1},"output":"$$ROOT"}}}},{$replaceRoot:{newRoot:"$root"}}])