[{messageId:1,reactions:[{"userid":"7212898122","reactionId":2,"date":"2022-05-12T18:55:49.943Z"},{"userid":"8171763225","reactionId":2,"date":"2022-05-12T19:27:34.324Z"},{"userid":"8782323232","reactionId":3,"date":"2022-05-12T18:55:49.943Z"}]},{messageId:2,reactions:[{"userid":"7212898122","reactionId":1,"date":"2022-05-12T18:55:49.943Z"},{"userid":"8171763225","reactionId":2,"date":"2022-05-12T19:27:34.324Z"},{"userid":"8782323232","reactionId":1,"date":"2022-05-12T18:55:49.943Z"}]},{messageId:3,reactions:[]}]

db.collection.aggregate([{$unwind:"$reactions"},{$group:{_id:{messageId:"$messageId",reactionId:"$reactions.reactionId"},count:{$sum:1}}},{$group:{_id:"$_id.messageId",count:{$push:{k:{$toString:"$_id.reactionId"},v:"$count"}}}},{$project:{messageId:"$_id",_id:0,count:{"$arrayToObject":"$count"}}}])