How to update _id field in MongoDB?

You can't directly update the _id field in MongoDB using standard update operations. The _id field is immutable once a document is created. However, you can achieve this by creating a new document with the desired _id and removing the old one.

Syntax

// Step 1: Find and store the document
var document = db.collection.findOne({field: "value"});

// Step 2: Change the _id
document._id = newIdValue;

// Step 3: Save as new document
db.collection.save(document);

// Step 4: Remove the old document
db.collection.remove({_id: oldIdValue});

Sample Data

Let's create a collection with a sample document ?

db.updatingIdFieldDemo.insertOne({"StudentName":"Chris"});

{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ce271bb36e8b255a5eee949")
}

db.updatingIdFieldDemo.find();

{ "_id" : ObjectId("5ce271bb36e8b255a5eee949"), "StudentName" : "Chris" }

Example: Update _id Field

Here's how to change the _id from ObjectId to a simple integer value ?

var myDocument = db.updatingIdFieldDemo.findOne({StudentName:"Chris"});
myDocument._id = 101;
db.updatingIdFieldDemo.save(myDocument);

WriteResult({ "nMatched" : 0, "nUpserted" : 1, "nModified" : 0, "_id" : 101 })

Now remove the old document with the original ObjectId ?

db.updatingIdFieldDemo.remove({_id:ObjectId("5ce271bb36e8b255a5eee949")});

WriteResult({ "nRemoved" : 1 })

Verify Result

db.updatingIdFieldDemo.find();

{ "_id" : 101, "StudentName" : "Chris" }

Key Points

• The _id field cannot be modified with $set or other update operators.
• The save() method creates a new document when the _id doesn't exist.
• Always remove the old document to avoid duplicates.

Conclusion

Updating the _id field requires a workaround: retrieve the document, modify its _id, save it as new, and remove the original. This effectively "updates" the _id by replacing the entire document.