Database
[{_id:100,streamer_id:"a01",viewers:4,title:"Event 1"},{_id:101,streamer_id:"a01",viewers:7,title:"Event 2"},{_id:103,streamer_id:"a02",viewers:3,title:"Stream"}]
Query
db.collection.aggregate([{$group:{_id:{streamer_id:"$streamer_id",},total_viewers:{$sum:"$viewers"}}},{$project:{_id:0,streamer_id:"$_id.streamer_id",total_viewers:1}}])