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Interviews / Databases / What is the `ObjectId` and what is it used for in MongoDB?

Questions

What is the difference between SQL and NoSQL databases?
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What is the `ObjectId` and what is it used for in MongoDB?
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What is GridFS and when would you use it in MongoDB?
Q C
What are capped collections and when would you use them in MongoDB?
Q C
How do you do data modeling in MongoDB?
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What are indexes and how do they work in MongoDB?
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What is the difference between a sparse and a partial index in MongoDB?
Q C
What is the aggregation framework and how do you use it in MongoDB?
Q C
How do you do replication in MongoDB?
Q C
How do you do sharding in MongoDB?
Q C
What are transactions and how do you use them in MongoDB?
Q C
What is the `explain()` method and what is it used for in MongoDB?
Q C

What is the `ObjectId` and what is it used for in MongoDB?

conceptual Core Concepts Interactive Quiz Code Examples

The Scenario

You are a backend engineer at a social media company. You are designing a new service that will store user-generated content, such as posts and comments.

You need to choose a primary key for your documents, and you are considering using the ObjectId data type.

The Challenge

Explain what the ObjectId is in MongoDB and why it is a good choice for a primary key. What are the different parts of an ObjectId, and what do they represent?

Wrong Approach

A junior engineer might not be aware of the `ObjectId` data type. They might try to use an auto-incrementing integer as the primary key, which is not a good choice for a distributed database like MongoDB.

Right Approach

A senior engineer would know that the `ObjectId` is the default primary key in MongoDB. They would be able to explain what an `ObjectId` is and why it is a good choice for a primary key. They would also be able to explain the different parts of an `ObjectId` and what they represent.

Step 1: Understand What an ObjectId Is

An ObjectId is a 12-byte BSON type that is the default primary key for documents in MongoDB.

Step 2: The Different Parts of an ObjectId

An ObjectId is made up of the following parts:

BytesDescription
4A timestamp, representing the ObjectId’s creation time.
3A machine identifier.
2A process identifier.
3A counter, starting with a random value.

The Benefits of Using an ObjectId

BenefitDescription
UniquenessObjectIds are unique across a sharded cluster.
SortabilityObjectIds are sortable by creation time.
PerformanceObjectIds are small and efficient to index.

When to Use a Custom Primary Key

In some cases, you might want to use a custom primary key instead of an ObjectId. For example, you might use a user’s email address as the primary key for a users collection.

However, in most cases, the ObjectId is a good choice for a primary key.

Practice Question

You want to be able to sort your documents by creation time. Which of the following would be the most appropriate?