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Aggregation Pipeline Operators

$toString (aggregation)

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  • Definition
  • Behavior
  • Example

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:

  • $convert

  • $dateToString

Behavior

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

Input Type
Behavior
Boolean
Returns the boolean value as a string.
Double
Returns the double value as a string.
Decimal
Returns the decimal value as a string.
Integer
Returns the integer value as a string.
Long
Returns the long value as a string.
ObjectId
Returns the ObjectId value as a hexadecimal string..
String
No-op. Returns the string value.
Date
Returns the date as a string.

The following table lists some conversion to string examples:

Example
Results
{$toString: true}
"true"
{$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.

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$topN (aggregation accumulator)