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Aggregation Operations

Reference

Operators

$toObjectId (aggregation)

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" } }

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.

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