db={"order":[{"_id":ObjectId("657b28e5866bd71e3746a4b7"),"status":"Completed","amount":750,"items":[{MedicineId:"6555f3ce551d5ce97ecd5767",Quantity:10},{MedicineId:"6573303a10b77858331909d5",Quantity:10},{MedicineId:"65733a6b1124e7849685b74e",Quantity:15}]}],"medicine":[{"_id":ObjectId("6555f3ce551d5ce97ecd5767"),"name":"Med 1","price":10},{"_id":ObjectId("6573303a10b77858331909d5"),"name":"Med 2","price":15},{"_id":ObjectId("65733a6b1124e7849685b74e"),"name":"Med 3","price":20},{"_id":ObjectId("65733a6b1124e7849685b75e"),"name":"Med 4","price":30}]}

db.medicine.aggregate([{$lookup:{from:"order",let:{medicineId:{$toString:"$_id"}},pipeline:[{$match:{$expr:{$and:[{$eq:["$status","Completed"]},{$in:["$$medicineId","$items.MedicineId"]}]}}},{$set:{items:{$filter:{input:"$items",cond:{$eq:["$$this.MedicineId","$$medicineId"]}}}}}],"as":"orders"}},{$match:{orders:{$ne:[]}}},{$project:{_id:0,medicineId:"$_id",medicineName:"$name",/** Adjust to your actual field name in Medicine model*/totalQuantity:{$sum:{$reduce:{input:"$orders",initialValue:[],in:{$concatArrays:["$$value","$$this.items.Quantity"]}}}},totalAmount:{$multiply:[{$sum:{$reduce:{input:"$orders",initialValue:[],in:{$concatArrays:["$$value","$$this.items.Quantity"]}}}},"$price"]}}}])