$ toDecimal（聚合）

db.orders.insert( [
   { _id: 1, item: "apple", qty: 5, price: 10 },
   { _id: 2, item: "pie", qty: 10, price: NumberDecimal("20.0") },
   { _id: 3, item: "ice cream", qty: 2, price: "4.99" },
   { _id: 4, item: "almonds",  qty: 5, price: 5 }
] )

// Define stage to add convertedPrice and convertedQty fields with the converted price and qty values

priceQtyConversionStage = {
   $addFields: {
      convertedPrice: { $toDecimal: "$price" },
      convertedQty: { $toInt: "$qty" },
   }
};

// Define stage to calculate total price by multiplying convertedPrice and convertedQty fields


totalPriceCalculationStage = {
   $project: { item: 1, totalPrice: { $multiply: [ "$convertedPrice", "$convertedQty" ] } }
};

db.orders.aggregate( [
   priceQtyConversionStage,
   totalPriceCalculationStage
])

{ "_id" : 1, "item" : "apple", "totalPrice" : NumberDecimal("50.0000000000000") }
{ "_id" : 2, "item" : "pie", "totalPrice" : NumberDecimal("200.0") }
{ "_id" : 3, "item" : "ice cream", "totalPrice" : NumberDecimal("9.98") }
{ "_id" : 4, "item" : "almonds", "totalPrice" : NumberDecimal("25.00000000000000") }