$toDecimal (aggregation)

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Definition定义

$toDecimal

New in version 4.0.版本4.0中的新功能。

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.

$toDecimal has the following syntax:语法如下所示：

{
   $toDecimal: <expression>
}

The $toDecimal takes any valid expression.

The $toDecimal is a shorthand for the following $convert expression:

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

See also参阅

$convert

Behavior行为

The following table lists the input types that can be converted to a decimal:

Input TypeBehavior
Boolean
Returns NumberDecimal("0") for false.
Returns NumberDecimal("1") for true.
Double Returns double value as a decimal.
Decimal No-op. Returns the decimal.
Integer Returns the int value as a decimal.
Long Returns the long value as a decimal.
String

Returns the numerical value of the string as a decimal.

The string value must be of a base10 numeric value (e.g. "-5.5", "123456").

You cannot convert a string value of a non-base10 number (e.g. "0x6400")

Date Returns the number of milliseconds since the epoch that corresponds to the date value.

The following table lists some conversion to decimal examples:

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

Example示例

Create a collection orders with the following documents:

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 }
] )

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:

// 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:

{ "_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") }

Note

If the conversion operation encounters an error, the aggregation operation stops and throws an error. To override this behavior, use $convert instead.