[{"_id":{"$oid":"60f66d177bb6ea476309b58b"},"__v":0,"employeeUserName":"mat@gmail.com","ended_at":{"$date":"2021-04-20T16:01:00.000Z"},"started_at":{"$date":"2021-04-20T09:12:00.000Z"},"totalHr":6.816666666666666},{"_id":{"$oid":"60f66d177bb6ea476309b58c"},"__v":0,"employeeUserName":"mat@gmail.com","ended_at":{"$date":"2021-04-20T16:01:00.000Z"},"started_at":{"$date":"2021-04-21T09:12:00.000Z"},"totalHr":7.816666666666666},{"_id":{"$oid":"60f66d177bb6ea476309b58d"},"__v":0,"employeeUserName":"mat@gmail.com","ended_at":{"$date":"2021-04-20T16:01:00.000Z"},"started_at":{"$date":"2021-03-21T09:12:00.000Z"},"totalHr":7.816666666666666}]

db.collection.aggregate([{$group:{_id:{employee:"$employeeUserName",day:{$dayOfMonth:"$started_at"},month:{$month:"$started_at"},year:{$year:"$started_at"}},totalHr:{$sum:"$totalHr"}}},{$group:{_id:{employee:"$_id.employee",month:"$_id.month",year:"$_id.year"},days:{$push:{day:"$_id.day",totalHr:"$totalHr"}},totalHr:{$sum:"$totalHr"}}},{$group:{_id:"$_id.employee",months:{$push:{number:"$_id.month",days:"$days",totalforMonth:"$totalHr"}}}},{$project:{_id:0,employee:"$_id",months:1}}])