[{"Object":{"Items":{"ItemDetail":[{"ItemNumber":3562,"ItemName":"LG-ICED LATTE"},{"ItemNumber":18419,"ItemName":"Sausage Biscuit",}],},"Payment":{"PaymentHeader":{"CashierNumber":23,"CashierName":"RVwGwYjpCsdYJg"},},}}]

db.collection.aggregate([{$unwind:"$Object.Items.ItemDetail"},{$group:{_id:{"CashierName":"$Object.Payment.PaymentHeader.CashierName","ItemName":"$Object.Items.ItemDetail.ItemName"},Count:{$count:{}}}},{$group:{_id:"$_id.CashierName",Items:{$push:{ItemName:"$_id.ItemName",Count:"$Count"}}}},{$project:{_id:0,CashierName:"$_id",Items:1}}])