$toDecimal (expression operator)（表达式运算符）

Definition定义

$toDecimal

Converts a value to a decimal. If the value cannot be converted to a decimal, $toDecimal errors. If the value is null or missing, $toDecimal returns null.将值转换为十进制。如果该值无法转换为十进制，则$toDecimal错误。如果该值为null或缺失，$toDecial将返回null。

$toDecimal has the following syntax:具有以下语法：

{
   $toDecimal: <expression>
}

The $toDecimal takes any valid expression.$toDecimal接受任何有效表达式。

The $toDecimal is a shorthand for the following $convert expression:$toDecimal是以下$convert表达式的简写：

{ $convert: { input: <expression>, to: "decimal" } }

Tip

$convert

Behavior行为

The following table lists the input types that can be converted to a decimal:下表列出了可以转换为十进制的输入类型：

The following table lists some conversion to decimal examples:下表列出了一些转换为十进制的示例：

Example示例

Create a collection orders with the following documents:使用以下文档创建集合orders：

db.orders.insertMany( [
   { _id: 1, item: "apple", qty: 5, price: 10 },
   { _id: 2, item: "pie", qty: 10, price: Decimal128("20.0") },
   { _id: 3, item: "ice cream", qty: 2, price: "4.99" },
   { _id: 4, item: "almonds",  qty: 5, price: 5 }
] )

The following aggregation operation on the orders collection converts the price to a decimal and the qty to an integer before calculating the total price:在计算总价之前，orders集合的以下聚合操作将price转换为小数，将qty转换为整数：

// Define stage to add convertedPrice and convertedQty fields with the converted price and qty values定义阶段以添加转换价格和转换数量字段，其中包含转换价格和数量值

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

// Define stage to calculate total price by multiplying convertedPrice and convertedQty fields定义阶段，通过将转换后的价格和转换后的数量字段相乘来计算总价


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

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

The operation returns the following documents:该操作返回以下文档：

{ _id: 1, item: 'apple', totalPrice: Decimal128("50") },
{ _id: 2, item: 'pie', totalPrice: Decimal128("200.0") },
{ _id: 3, item: 'ice cream', totalPrice: Decimal128("9.98") },
{ _id: 4, item: 'almonds', totalPrice: Decimal128("25") }