In MongoDB, each document stored in a collection is uniquely identified by a field called _id. This _id field serves as the primary key for the document and is important for ensuring document uniqueness and efficient retrieval. But have you ever wondered how these _id values are generated in MongoDB?

In this article, we’ll explore the Complexities of MongoDB ID generation by covering concepts, and examples to provide a comprehensive understanding.

What is the _id Field in MongoDB?

• The _id field serves as the primary key for documents in a collection, ensuring each document has a unique identifier.
• MongoDB guarantees that each _id value is unique within a collection.
• If we don’t provide an _id field when inserting a document, MongoDB will automatically generate one for us . The default generation mechanism uses ObjectId.
• The most common type of _id value is ObjectId, which is a 12–byte identifier consisting of a timestamp, a random value, and an incrementing counter.
• While MongoDB provides automatic _id generation, we can also specify custom _id values during document insertion, allowing us to use other data types such as strings or integers.
• The _id field is indexed by default, which can impact performance, especially when using custom _id values or very large collections.
• MongoDB provides efficient ways to query documents by their _id values, making it easy to retrieve specific document

Generating an ObjectId

Let’s learn about how MongoDB generates an ObjectId. We can use the MongoDB shell to demonstrate this process.

Example of Generating an ObjectId

Open the MongoDB shell by running the Mongo command in your terminal or command prompt. Then, execute the following command to generate an ObjectId:

ObjectId()

Output:

ObjectId("62170c5af3d27e919f30b100")

In this example, MongoDB generates a new ObjectId with a unique value.

Custom _id Values

While MongoDB automatically generates ObjectId values for the _id field if one is not provided, we can also specify custom _id values during document insertion. This allows us to use other data types such as strings, integers, or even other objects as _id values.

Example of Specifying a Custom _id

db.users.insertOne({ _id: "user123", name: "John Doe", age: 30 })

In this example, we’re inserting a new document into the users collection with a custom _id value of “user123”.

Output:

{ "_id" : "user123", "name" : "John Doe", "age" : 30 }

Impact of _id Generation on Performance

MongoDB’s default _id generation strategy using ObjectId values provides several benefits, including:

• Uniqueness: ObjectId values are globally unique, ensuring document uniqueness across collections and databases.
• Scalability: ObjectId values are generated without the need for centralized coordination, making them suitable for distributed systems.
• Performance: The generation of ObjectId values is efficient, even at high write volumes.

Conclusion

Overall, Understanding how _id values are generated in MongoDB is essential for effectively managing and querying documents within collections. By default, MongoDB uses ObjectId values to uniquely identify documents, but you can also specify custom _id values to meet specific requirements. In this article, we explored the process of _id generation in MongoDB, covering concepts, examples, and outputs to provide beginners with a comprehensive understanding of this fundamental aspect of MongoDB document management. As you continue to work with MongoDB, mastering the generation and usage of _id values will empower you to build robust and efficient database solutions.