Database
[{"key":"a","time":ISODate("2021-10-20T00:00:00.000Z")},{"key":"a","time":ISODate("2020-10-20T00:00:00.000Z")},{"key":"b","time":ISODate("2020-10-20T00:00:00.000Z")},{"key":"b","time":ISODate("2019-10-20T00:00:00.000Z")}]
Query
db.collection.aggregate([{"$sort":{"key":1,"time":-1}},{$group:{_id:"$key","data":{$first:"$$ROOT"}}},{"$replaceWith":"$data"}])