[{"_id":ObjectId("611ba6d26aaaf2446f7fc45e"),"hashtags":["test1","test2"],"hour":ISODate("2021-08-16T04:00:00.000Z"),"username":"username1","lang":"fr",},{"_id":ObjectId("611ba6d26aaaf2446f7fc45f"),"hashtags":["test1"],"hour":ISODate("2021-08-09T04:00:00.000Z"),"lang":"fr","username":"username1"},{"_id":ObjectId("611ba6d26aaaf2446f7fc460"),"hashtags":["test2","test3"],"hour":ISODate("2021-08-16T04:00:00.000Z"),"lang":"en","username":"username2"},{"_id":ObjectId("611ba6d26aaaf2446f7fc462"),"hashtags":["test2","test3"],"hour":ISODate("2021-08-16T04:00:00.000Z"),"lang":"en","username":"username2"}]

db.collection.aggregate([{"$group":{"_id":{"hour":"$hour"},"nbTweets":{"$sum":1},"nbRetweets":{"$sum":"$retweetCount"},"nbQuotes":{"$sum":"$retweetCount"},"nbLikes":{"$sum":"$likeCount"},usernames:{"$push":"$username"},hashtags:{"$push":"$hashtags"},languages:{"$push":"$lang"},},},{$addFields:{hashtags:{$reduce:{input:"$hashtags",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}},}},{"$sort":{"_id.hour":-1},}])