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$toString (aggregation)

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Definition

$toString

Converts a value to a string. If the value cannot be converted to a string, $toString errors. If the value is null or missing, $toString returns null.

$toString has the following syntax:

{
   $toString: <expression>
}

The $toString takes any valid expression.

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

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

Tip

See also:

Behavior

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

Input TypeBehavior
BooleanReturns the boolean value as a string.
DoubleReturns the double value as a string.
DecimalReturns the decimal value as a string.
IntegerReturns the integer value as a string.
LongReturns the long value as a string.
ObjectIdReturns the ObjectId value as a hexadecimal string..
StringNo-op. Returns the string value.
DateReturns the date as a string.

The following table lists some conversion to string examples:

ExampleResults
{$toString: true}"true"
{$toString: false}"false"
{$toString: 2.5}"2.5"
{$toString: NumberInt(2)}"2"
{$toString: NumberLong(1000)}"1000"
{$toString: ObjectId("5ab9c3da31c2ab715d421285")}"5ab9c3da31c2ab715d421285"
{$toString: ISODate("2018-03-27T16:58:51.538Z")}"2018-03-27T16:58:51.538Z"

Example

Create a collection orders with the following documents:

db.orders.insertMany( [
   { _id: 1, item: "apple",  qty: 5, zipcode: 93445 },
   { _id: 2, item: "almonds", qty: 2, zipcode: "12345-0030" },
   { _id: 3, item: "peaches",  qty: 5, zipcode: 12345 },
] )

The following aggregation operation on the orders collection converts the zipcode to string before sorting by the string value:

// Define stage to add convertedZipCode field with the converted zipcode value


zipConversionStage = {
   $addFields: {
      convertedZipCode: { $toString: "$zipcode" }
   }
};


// Define stage to sort documents by the converted zipcode


sortStage = {
   $sort: { "convertedZipCode": 1 }
};


db.orders.aggregate( [
  zipConversionStage,
  sortStage
] )

The operation returns the following documents:

{
  _id: 3,
  item: 'peaches',
  qty: 5,
  zipcode: 12345,
  convertedZipCode: '12345'
},
{
  _id: 2,
  item: 'almonds',
  qty: 2,
  zipcode: '12345-0030',
  convertedZipCode: '12345-0030'
},
{
  _id: 1,
  item: 'apple',
  qty: 5,
  zipcode: 93445,
  convertedZipCode: '93445'
}

Note

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