Cast to ObjectId failed for value in MongoDB?

The "Cast to ObjectId failed" error occurs when MongoDB cannot convert a string value to a valid ObjectId. This happens when the string is not a valid 24-character hexadecimal ObjectId format. Use the $toObjectId operator in aggregation pipelines to safely convert string values to ObjectId.

Syntax

db.collection.aggregate([
    {
        $addFields: {
            "newField": { $toObjectId: "$stringField" }
        }
    }
]);

Sample Data

Let us create a collection with a string _id that we want to convert to ObjectId ?

db.demo460.insertOne({
    "_id": "5ab9cbfa31c2ab715d42129e"
});

{
    "acknowledged": true,
    "insertedId": "5ab9cbfa31c2ab715d42129e"
}

Display the document to verify the string _id ?

db.demo460.find();

{ "_id": "5ab9cbfa31c2ab715d42129e" }

Example: Convert String to ObjectId

Use $toObjectId to convert the string _id to a proper ObjectId ?

db.demo460.aggregate([
    {
        $addFields: {
            "convertedId": { $toObjectId: "$_id" }
        }
    }
]);

{
    "_id": "5ab9cbfa31c2ab715d42129e",
    "convertedId": ObjectId("5ab9cbfa31c2ab715d42129e")
}

Key Points

• The string must be exactly 24 characters and contain only hexadecimal characters (0-9, a-f).
• Use $toObjectId in aggregation pipelines for safe conversion.
• Invalid strings will cause the aggregation to fail with a cast error.

Conclusion

Use the $toObjectId operator in MongoDB aggregation pipelines to convert valid 24-character hexadecimal strings to ObjectId. Ensure the string format is correct to avoid cast failures.