[{_id:1,sender:"foo",messages:[{_id:10,text:"hello",recipientCount:3,sentAt:"2019-10"},{_id:11,text:"hello",recipientCount:3,sentAt:"2019-10"},{_id:12,text:"hello",recipientCount:3,sentAt:"2019-10"},]},{_id:2,sender:"bar",messages:[{_id:13,text:"hello",recipientCount:3,sentAt:"2018-10"},{_id:14,text:"hello",recipientCount:3,sentAt:"2018-10"},{_id:15,text:"hello",recipientCount:3,sentAt:"2018-10"},]},{_id:3,sender:"foo",messages:[{_id:16,text:"hello",recipientCount:13,sentAt:"2020-10"},{_id:17,text:"hello",recipientCount:13,sentAt:"2020-10"},{_id:18,text:"hello",recipientCount:13,sentAt:"2020-10"},]},{_id:4,sender:"foo",messages:[{_id:19,text:"hello",recipientCount:3,sentAt:"2021-10"},{_id:110,text:"hello",recipientCount:3,sentAt:"2021-10"},{_id:111,text:"hello",recipientCount:3,sentAt:"2021-10"},]},{_id:5,sender:"bar",messages:[{_id:112,text:"hello",recipientCount:1,sentAt:"2021-4"},{_id:113,text:"hello",recipientCount:1,sentAt:"2021-4"},{_id:114,text:"hello",recipientCount:1,sentAt:"2021-4"},]},{_id:6,sender:"foo",messages:[{_id:115,text:"hello",recipientCount:4,sentAt:"2020-8"},{_id:116,text:"hello",recipientCount:4,sentAt:"2020-8"},{_id:117,text:"hello",recipientCount:4,sentAt:"2020-8"},]},{_id:7,sender:"cap",messages:[{_id:118,text:"hello",recipientCount:7,sentAt:"2018-6"},{_id:119,text:"hello",recipientCount:7,sentAt:"2018-6"},{_id:120,text:"hello",recipientCount:7,sentAt:"2018-6"},]},{_id:8,sender:"cap",messages:[{_id:121,text:"hello",recipientCount:12,sentAt:"2019-11"},{_id:122,text:"hello",recipientCount:12,sentAt:"2019-11"},{_id:123,text:"hello",recipientCount:12,sentAt:"2019-11"},]},{_id:9,sender:"foo",messages:[{_id:124,text:"hello",recipientCount:2,sentAt:"2020-12"},{_id:125,text:"hello",recipientCount:2,sentAt:"2020-12"},{_id:126,text:"hello",recipientCount:2,sentAt:"2020-12"},]},{_id:10,sender:"foo",messages:[{_id:127,text:"hello",recipientCount:1,sentAt:"2021-1"},{_id:128,text:"hello",recipientCount:1,sentAt:"2021-1"},{_id:129,text:"hello",recipientCount:1,sentAt:"2021-1"},]},{_id:11,sender:"cap",messages:[{_id:130,text:"hello",recipientCount:2,sentAt:"2019-2"},{_id:131,text:"hello",recipientCount:2,sentAt:"2019-2"},{_id:132,text:"hello",recipientCount:2,sentAt:"2019-2"},]},]

db.collection.aggregate([{"$unwind":"$messages"},{"$group":{"_id":{sender:"$sender",sentAt:"$messages.sentAt"},"count":{"$sum":"$messages.recipientCount"}}},{"$project":{"sender":"$_id.sender","sentAt":"$_id.sentAt","count":1,"_id":0}}])