[{"_id":1,"invoiceId":"ABCDEFG","username":"randomUser","amount":80,"createdAt":ISODate("2022-04-18T06:59:07.836Z")},{"_id":2,"invoiceId":"ABCDEFG","username":"randomUser","amount":70,"createdAt":ISODate("2022-04-19T06:59:07.836Z")},{"_id":3,"invoiceId":"ABCDEFG","username":"randomUser","amount":55,"createdAt":ISODate("2022-05-18T06:59:07.836Z")},]

db.collection.aggregate([{"$group":{"_id":{month:{"$month":"$createdAt"},year:{"$year":"$createdAt"}},"totalSpending":{$sum:"$amount"}}},{"$project":{"totalSpending":1,"period":{"$concat":[{$toString:"$_id.year"},"-",{$toString:"$_id.month"}]},"_id":0}}])