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Aggregation Pipeline Operators

$toObjectId (aggregation)

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  • Definition
  • Behavior
  • Example

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

See also:

$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.insert( [
   { _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.

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$toString (aggregation)