$toString (aggregation)

Definition

• $toString

New in version 4.0.

Converts a value to a string. If the value cannot be convertedto a string, $toString errors. If the value is null ormissing, $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" } }

See also

$convert and $dateToString

Behavior

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

The following table lists some conversion to string examples:

Example

Create a collection orders with the following documents:

db.orders.insert( [
   { _id: 1, item: "apple",  qty: 5, zipcode: 12345 },
   { _id: 2, item: "pie",  qty: 10, zipcode: 11111 },
   { _id: 3, item: "ice cream",  zipcode: "12345" },
   { _id: 4, item: "almonds", qty: 2, zipcode: "12345-0030" },
])

The following aggregation operation on the orders collectionconverts 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" : 2, "item" : "pie", "qty" : 10, "zipcode" : 11111, "convertedZipCode" : "11111" }
{ "_id" : 1, "item" : "apple", "qty" : 5, "zipcode" : 12345, "convertedZipCode" : "12345" }
{ "_id" : 3, "item" : "ice cream", "zipcode" : "12345", "convertedZipCode" : "12345" }
{ "_id" : 4, "item" : "almonds", "qty" : 2, "zipcode" : "12345-0030", "convertedZipCode" : "12345-0030" }

Note

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