MongoDB: How to use custom primary key (e.g., my_id instead of _id)

Overview

MongoDB, a leading NoSQL database, is known for its flexibility, scalability, and rich feature set. One of the customization features it offers is the ability to use a custom primary key instead of the default _id field. In this tutorial, we’ll explore the reasons for using a custom primary key, how to implement it, and some advanced considerations to keep in mind.

MongoDB’s Default Primary Key

MongoDB automatically assigns a unique identifier to the _id field of each document if it is not explicitly provided. This ID is of the ObjectId type, which is designed to be unique across collections. While this auto-generation feature is convenient, there are scenarios where a custom primary key is preferable or necessary.

Why Use a Custom Primary Key?

There are several reasons you might want to use a custom primary key in MongoDB:

• Application-specific identifiers: Your application may require using a domain-specific identifier, like a user ID or product SKU, as the primary key.
• Improved readability: ObjectIds, while unique, are not necessarily human-readable or meaningful. Custom keys can offer more insight at a glance.
• Integration requirements: When integrating with existing systems, matching MongoDB documents to external records can be easier with a custom primary key.

Implementing a Custom Primary Key

To use a custom primary key, you simply need to ensure that your documents contain a unique field that you designate as the primary key. However, ensure this field is always unique and indexed to maintain performance.

Basic Example

{
  "my_id": "user123",
  "name": "John Doe",
  "email": "[email protected]"
}

In this example, my_id serves as our custom primary key. Notice how it replaces the default _id field. Ensure that my_id is unique across all documents in the collection.

Ensuring Uniqueness and Performance

Create an index on your custom primary key to ensure uniqueness and maintain query performance:

db.collection.createIndex({"my_id": 1}, {unique: true});

This index prevents duplicate my_id values and optimizes lookup speeds.

Advanced Considerations

While opting for a custom primary key, consider the following advanced aspects to ensure system stability and performance:

• Data Migration: If you’re transitioning to a custom primary key, ensure a smooth migration process to prevent data inconsistency.
• Scalability: Evaluate the scalability of your custom key choice, particularly if you’re using a value that could potentially see format changes.
• Replication and Sharding: Consider the implications on replication and sharding. Ensure your custom primary key supports your database cluster’s scalability and redundancy objectives.

Duplicate Prevention Technique

When using a non-ObjectId _id, implementing additional checks or using database features like upserts are essential for preventing duplicates:

db.collection.update(
  { "my_id": "user123" },
  { "$set": { "name": "John Updated" } },
  { "upsert": true }
);

This operation checks for a document with my_id equals “user123”. If found, it updates the name; if not, it inserts the new document, effectively preventing duplicates.

Conclusion

Utilizing a custom primary key in MongoDB can unlock several benefits, from enhanced readability to smoother integration with external systems. By carefully designing your key strategy and properly configuring indexes and data integrity measures, you can successfully implement a custom primary key to meet your application’s specific needs.