[{"products":[{"_id":{"$oid":"5ffd0a8f6273740017cc5fca"},"name":"Banana","price":65,"createdAt":{"$date":"2021-01-12T02:33:51.648Z"},"updatedAt":{"$date":"2021-01-12T02:33:51.648Z"},"quantity":3},{"_id":{"$oid":"5ffd09326273740017cc5fb3"},"name":"Apple","price":79,"createdAt":{"$date":"2021-01-12T02:28:02.412Z"},"updatedAt":{"$date":"2021-01-12T02:28:02.412Z"},"quantity":2}]}]

db.collection.aggregate({$addFields:{"products":{$map:{input:"$products",as:"p",in:{"$mergeObjects":[{total:{$multiply:["$$p.price","$$p.quantity"]}},"$$p"]}}}}})