MongoDB - ObjectId

MongoDB - Understanding ObjectId

Every document in a MongoDB collection requires a unique _id field that acts as its primary key. If you don’t provide one, MongoDB automatically generates a unique ObjectId for the _id field when you insert the document.

An ObjectId is a 12-byte BSON type, meticulously crafted to ensure a high probability of uniqueness across different machines and at different times. It is composed of:

• A 4-byte timestamp value, representing the ObjectId’s creation, measured in seconds since the Unix epoch.
• A 5-byte random value (this has replaced the former machine + process ID for better distribution).
• A 3-byte incrementing counter, initialized to a random value.

This structure provides a default, sortable, and globally unique identifier for your documents.

Working with ObjectId

Generating a New ObjectId

You can generate a new ObjectId on demand in mongosh or your application driver.

// In mongosh
> const newId = new ObjectId()
> newId
ObjectId("615cc1b3f72d5c4b1e9b1e2a")

Extracting the Timestamp

Because the creation time is embedded within the ObjectId, you can extract it without needing a separate createdAt field (though explicit timestamps are often clearer). Use the getTimestamp() method:

> const myId = new ObjectId("615cc1b3f72d5c4b1e9b1e2a")
> myId.getTimestamp()
ISODate("2021-10-05T19:33:07.000Z")

This feature makes ObjectIds naturally sortable by creation time.

Converting ObjectId to String

Often, you need to use the ObjectId’s value as a string, for example in a REST API URL, in a log file, or as a key in a JSON object being sent to a web client.

Modern Conversion Methods

Use the .toString() or .toHexString() methods to get the 24-character hexadecimal string representation. The legacy .str property is deprecated.

> const myId = new ObjectId()
ObjectId("615cc204f72d5c4b1e9b1e2b")

> myId.toString()
'615cc204f72d5c4b1e9b1e2b'

> myId.toHexString()
'615cc204f72d5c4b1e9b1e2b'

Practical Application and Best Practices

• Immutability: The _id field is immutable. You cannot update the _id of a document after it has been inserted. If you need to change it, you must create a new document with the new _id and remove the old one.
• Custom _id: You can provide your own value for _id, as long as it’s unique within the collection. This is common when using a natural primary key, like a user’s email or a product SKU. However, these custom keys are not guaranteed to have the same useful sorting properties as an ObjectId.
• Client-Side Generation: ObjectIds can be generated by the client-side driver before a document is even sent to the database. This allows you to know a document’s ID before inserting it, which can simplify application logic.