[{"_id":"oI","msg":"Hello","ts":ISODate("2019-09-20T01:01:43.749Z")},{"_id":"z","msg":"can you hear me?","ts":ISODate("2019-09-20T01:01:46.569Z"),"tcount":3},{"_id":"wG","msg":"Undsen bichver 2","ts":ISODate("2019-09-20T02:00:58.254Z")},{"_id":"BF","msg":"Undsen bichver 3","ts":ISODate("2019-09-20T02:01:06.784Z")},{"_id":"Jq","msg":"Undsen bichver 4","ts":ISODate("2019-09-20T02:01:11.785Z")},{"_id":"oP","parentId":"z","msg":"Test 1","ts":ISODate("2019-09-20T01:02:06.860Z")},{"_id":"Mq","parentId":"z","msg":"Test 2","ts":ISODate("2019-09-20T01:05:37.173Z")},{"_id":"Lq","parentId":"z","msg":"Test 3","ts":ISODate("2019-09-20T02:01:24.370Z")}]

db.collection.aggregate([{$addFields:{sortBy:{$ifNull:["$parentId","$_id"]}}},{$sort:{sortBy:-1,parentId:1}}])