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$toDecimal (aggregation)

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Definition定义

$toDecimal

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

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

{
   $toDecimal: <expression>
}

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

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

See also: 另请参阅：

$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：

Input Type输入类型Behavior行为
BooleanReturns Decimal128("0") for false.
Returns Decimal128("1") for true.
DoubleReturns double value as a decimal.以十进制形式返回双值。
DecimalNo-op. Returns the decimal.没有。返回小数。
IntegerReturns the int value as a decimal.以十进制形式返回int值。
LongReturns the long value as a decimal.以十进制形式返回长值。
StringReturns the numerical value of the string as a decimal.以十进制形式返回字符串的数值。
The string value must be of a base 10 numeric value (e.g. "-5.5", "123456").字符串值必须是以10为基数的数值（例如"-5.5"、"123456"）。
You cannot convert a string value of a non-base 10 number (e.g. "0x6400") 不能转换非基数10的字符串值（例如"0x6400"）
DateReturns the number of milliseconds since the epoch that corresponds to the date value.返回自与日期值对应的epoch以来的毫秒数。

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Results结果
{$toDecimal: true}Decimal128("1")
{$toDecimal: false}Decimal128("0")
{$toDecimal: 2.5}Decimal128("2.50000000000000")
{$toDecimal: NumberInt(5)}Decimal128("5")
{$toDecimal: NumberLong(10000)}Decimal128("10000")
{$toDecimal: "-5.5"}Decimal128("-5.5")
{$toDecimal: ISODate("2018-03-27T05:04:47.890Z")}Decimal128("1522127087890")

Example实例

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: NumberDecimal("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集合上的以下聚合操作将price转换为十进制，将qty转换为整数：

// 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
] )

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") }
Note

If the conversion operation encounters an error, the aggregation operation stops and throws an error. 如果转换操作遇到错误，聚合操作将停止并引发错误。To override this behavior, use $convert instead.若要覆盖此行为，请改用$convert。