[{"name":"John Doe","email":"john@doe.com","progress":[{"_id":"610be25ae20ce4872b814b24","challenge":"60f9629edd16a8943d2cab9b","date_unlocked":"2020-08-05T12:15:32.129+00:00","completed":true,"date_completed":"2021-08-06T12:15:32.129+00:00"},{"_id":"611be24ae32ce4772b814b32","challenge":"60g6723efd44a6941l2cab81","date_unlocked":"2021-08-06T12:15:32.129+00:00","completed":true,"date_completed":"2021-08-07T12:15:32.129+00:00"}],"date":"2021-08-04T13:06:34.129+00:00"}]

db.collection.aggregate([{"$unwind":{"path":"$progress"}},{"$sort":{"progress.date_unlocked":-1}},{"$limit":1},{"$project":{"_id":0,"latestChallenge":"$progress.challenge"}}])