$toObjectId (expression operator)

Definition

$toObjectId

Converts a value to an ObjectId(). If the value cannot be converted to an ObjectId, $toObjectId errors. If the value is null or missing, $toObjectId returns null.

$toObjectId has the following syntax:

{   $toObjectId: <expression>}

The $toObjectId takes any valid expression.

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

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

Tip

$convert

Behavior

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

Input Type	Behavior
String	Returns an ObjectId for the hexadecimal string of length 24. You cannot convert a string value that is not a hexadecimal string of length 24.

The following table lists some conversion to date examples:

Example	Results
`{$toObjectId: "5ab9cbfa31c2ab715d42129e"}`	ObjectId("5ab9cbfa31c2ab715d42129e")
`{$toObjectId: "5ab9cbfa31c2ab715d42129"}`	Error

Example

Create a collection orders with the following documents:

db.orders.insertMany( [   { _id: "5ab9cbe531c2ab715d42129a", item: "apple", qty: 10 },   { _id: ObjectId("5ab9d0b831c2ab715d4212a8"), item: "pie", qty: 5 },   { _id: ObjectId("5ab9d2d331c2ab715d4212b3"), item: "ice cream", qty: 20 },   { _id: "5ab9e16431c2ab715d4212b4", item: "almonds", qty: 50 },] )

The following aggregation operation on the orders collection converts the _id to ObjectId before sorting by the value:

// Define stage to add convertedId field with converted _id value
idConversionStage = {   $addFields: {      convertedId: { $toObjectId: "$_id" }   }};
// Define stage to sort documents by the converted qty values
sortStage = {   $sort: { "convertedId": -1 }};

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

The operation returns the following documents:

{  _id: '5ab9e16431c2ab715d4212b4',  item: 'almonds',  qty: 50,  convertedId: ObjectId("5ab9e16431c2ab715d4212b4")},{  _id: ObjectId("5ab9d2d331c2ab715d4212b3"),  item: 'ice cream',  qty: 20,  convertedId: ObjectId("5ab9d2d331c2ab715d4212b3")},{  _id: ObjectId("5ab9d0b831c2ab715d4212a8"),  item: 'pie',  qty: 5,  convertedId: ObjectId("5ab9d0b831c2ab715d4212a8")},{  _id: '5ab9cbe531c2ab715d42129a',  item: 'apple',  qty: 10,  convertedId: ObjectId("5ab9cbe531c2ab715d42129a")}

Note

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

Back

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