Working with ObjectId and _id

                    Working with ObjectId and _id in MongoDB

Introduction

As soon as you begin coding with MongoDB, one of the first things you'll find in each document is the enigmatic appearing field named _id. It's not merely an arbitrary string – it is an identifier and, as default, is generated using a special type named ObjectId. Whether you're new to MongoDB or you're looking to move beneath the surface, learning how _id and ObjectId work is something that will make you a more effective user of MongoDB.

Explanation

What is _id?
In MongoDB, each document has to have an _id field. Imagine _id as a primary key – it makes the document unique, so MongoDB can figure out how to retrieve, update, or remove the correct one.

What is ObjectId?
ObjectId is a 12-byte id that MongoDB automatically creates if you don't supply a custom _id. It will look something like this:

ObjectId("64ffbae5314a2187253fc5c1")

Each component of this ID has significance:

4 bytes: timestamp
3 bytes: machine ID
2 bytes: process ID
3 bytes: counter (preventing duplication)

This structure creates ObjectIds:

Globally unique
Time-sortable (on creation time)

Procedure

Here's how you work with _id and ObjectId in MongoDB:

1. Displaying _id in a document
When you insert a document with no _id, MongoDB automatically adds it.

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

{
"_id": ObjectId("64ffbae5314a2187253fc5c1"),
"name": "Alice"
}

2. Using a custom _id
You can specify your own _id if you prefer:

db.users.insertOne({ _id: "user123", name: "Bob" })

Every _id has to be distinct.

3. Querying with ObjectId
You have to use ObjectId() in order to query it properly.

db.users.find({ _id: ObjectId("64ffbae5314a2187253fc5c1") })

4. Creating ObjectId manually
You can create one programmatically:

const id = new ObjectId();

Or even pull out its timestamp:

id.getTimestamp(); // returns ISODate

Future Scope

Manipulating _id and ObjectId might seem easy, but they are behind most sophisticated MongoDB operations. Here's how they contribute heavily towards the future:

• Sharding: MongoDB employs _id to split documents into shards.
• Time-based queries: With a timestamp in ObjectId, you can query documents by creation time.

• Optimized indexing: MongoDB utilizes _id as an index by default – excellent for performance.
• Custom ID generation: In systems of a large scale, developers can even create their own UUIDs or hash-based _ids for security or scaling purposes.

Conclusion

The _id field and ObjectId are not merely random IDs – they guarantee uniqueness, enable time tracking, and are deeply linked to MongoDB's performance and scaling mechanisms. Whether you have MongoDB generate them or generate your own, knowing how they work helps you develop better, more solid applications.

Vedansh Shah

University: Shree Balaji University, Pune

School: School of Computer Studies

Course: BCA (Bachelor of Computer Applications)

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