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db.collection.aggregate([{"$addFields":{"latestChallenge":{"$arrayElemAt":[{"$reduce":{"input":"$progress","initialValue":["0",""],"in":{"$let":{"vars":{"info":"$$value","progress":"$$this"},"in":{"$cond":[{"$gt":["$$progress.date_unlocked",{"$arrayElemAt":["$$info",0]}]},[{"$arrayElemAt":["$$info",0]},"$$progress.challenge"],"$$info"]}}}}},1]}}},{"$project":{"_id":0,"latestChallenge":1}},])