Auto-generated _id behavior in MongoDB

Introduction

MongoDB automatically creates a unique _id for each document to identify it easily without extra work.

When you insert a new document and don't provide an _id, MongoDB creates one for you.

When you want to ensure each document has a unique identifier without manually generating it.

When you want to quickly find or update a document by its unique _id.

When you want to avoid duplicate documents in a collection.

When you want to link documents using their unique _id values.

Syntax

MongoDB

db.collection.insertOne({ field1: value1, field2: value2 })

If you do not specify the _id field, MongoDB adds it automatically.

The auto-generated _id is an ObjectId, which is a 12-byte unique value.

Examples

Inserts a new user document. MongoDB creates a unique _id automatically.

MongoDB

db.users.insertOne({ name: "Alice", age: 30 })

Inserts a user with a custom _id instead of auto-generated one.

MongoDB

db.users.insertOne({ _id: "customID123", name: "Bob" })

Finds a document by its auto-generated ObjectId _id.

MongoDB

db.users.findOne({ _id: ObjectId("507f1f77bcf86cd799439011") })

Sample Program

This inserts a product without specifying _id. MongoDB creates a unique _id automatically. Then it finds and shows the inserted document using that _id.

MongoDB

use testdb

// Insert a document without _id
var result = db.products.insertOne({ name: "Pen", price: 1.5 })

// Show the inserted document with auto-generated _id
var doc = db.products.findOne({ _id: result.insertedId })
doc

Important Notes

The ObjectId contains a timestamp, machine id, process id, and a counter to ensure uniqueness.

You can provide your own _id, but it must be unique in the collection.

Trying to insert a document with a duplicate _id will cause an error.

Summary

MongoDB auto-generates a unique _id for each document if you don't provide one.

The _id is an ObjectId by default, which helps uniquely identify documents.

You can use the _id to quickly find, update, or delete documents.