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$toDecimal (aggregation)

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  • Definition
  • Behavior
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$toDecimal

New in version 4.0.

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:

{
$toDecimal: <expression>
}

The $toDecimal takes any valid expression.

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

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

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

Input Type
Behavior
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 base 10 numeric value (e.g. "-5.5", "123456").

You cannot convert a string value of a non-base 10 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
Results
{$toDecimal: true}
NumberDecimal("1")
{$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")

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.

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