Database
[{"_id":"608642db80a36336946620aa","cards":[{"stats":{"reviewDate":"1985-03-26T04:34:17.425Z",},"_id":"6086430080a36336946620ab","front":"front","back":"back"},{"stats":{"reviewDate":"2021-05-26T04:34:17.425Z",},"_id":"6086430080a36336946620ab","front":"front","back":"back"},{"stats":{"reviewDate":"2021-04-26T04:34:17.425Z",},"_id":"6086430080a36336946620ab","front":"front","back":"back"}]}]
Query
db.collection.aggregate([{$unwind:"$cards"},{$sort:{"cards.stats.dateCreated":-1}},{$limit:1},{$replaceRoot:{"newRoot":"$cards"}}])