The _id Field and ObjectId

The _id Field and ObjectId

Every document in a collection must have a unique _id field - it acts as the document's primary key. If you don't provide one, MongoDB generates an ObjectId for you.

Auto-generated _id

db.posts.insertOne({ title: "Hello" })
// result: { acknowledged: true, insertedId: ObjectId("507f1f77bcf86cd799439011") }

If your document has no _id, MongoDB adds one with a unique ObjectId. If you supply your own _id, it must be unique in the collection:

db.posts.insertOne({ _id: 10, title: "Inception" })

What's inside an ObjectId?

An ObjectId is 12 bytes:

1. 4-byte timestamp - seconds since the Unix epoch (creation time)
2. 5-byte random value - unique per machine and process
3. 3-byte incrementing counter - a per-process counter

Reading the creation time

Because the first 4 bytes are a timestamp, you can recover when a document was created:

ObjectId("507f1f77bcf86cd799439011").getTimestamp()

Good to know

• ObjectIds are unique and sortable roughly by creation time.
• The _id index is automatically created and unique - you can't drop it.
• In a projection, _id is included unless you explicitly exclude it.

TL;DR

• Every document needs a unique _id (primary key).
• If missing, MongoDB generates an ObjectId.
• ObjectId = 4-byte timestamp + 5-byte random + 3-byte counter.