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Discover the best practices for storing relationships in MongoDB. Learn when to use `ObjectId` or `String` for optimal database performance and error handling.
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How to Store Relationships in MongoDB: The Best Approach to Using ObjectId and String

When working with MongoDB, a common challenge developers face is how to effectively store relationships between different collections, such as products and categories. Suppose you have a products collection, and you need to store a reference to a category. This leads to a crucial question: How should the category_id be represented in your product documents?

In this post, we will explore two approaches for representing the category_id field and provide clear guidance on which method to choose based on your use case.

The Issue at Hand

Let’s say your products collection looks like this:

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You want to add the category_id, and you have two potential approaches to consider:

Approach 1: Using ObjectId

[[See Video to Reveal this Text or Code Snippet]]

Approach 2: Using String

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Choosing the Right Approach

When to Use ObjectId

If you frequently use $lookup operations to join documents from different collections, then embedding the category_id as an ObjectId is highly recommended.

Benefits of using ObjectId:

Seamless joins: Using ObjectId allows MongoDB to efficiently execute join queries.

Direct compatibility: No need for format conversion when executing aggregation queries involving lookups.

When to Use String

If the application or programming driver you are using does not support ObjectId, you may consider storing the category_id as a string.

Considerations for using String:

Easier integration: Pasting string values into REST APIs can be straightforward since no type conversion is needed for most frameworks.

Avoiding complex conversions: If your application faces frequent type conversion from string to ObjectId, it can become cumbersome and error-prone, especially since ObjectId has strict validation rules.

Real-World Example

In practice, at my current workplace, we opted to store category_id as ObjectId due to the volume of $lookup operations required. While this approach simplifies data retrieval via aggregation, it created an initial friction in handling data conversions in our backend.

Error Handling

When you convert from String to ObjectId, you often run the risk of errors, which require extra error handling in your code base. Unlike ObjectId to string conversions—which are generally safer—string to ObjectId can fail if the string does not conform with the ObjectId structure.

Conclusion

Ultimately, the best approach for storing relationships in MongoDB hinges on your specific application requirements. If you expect heavy database querying and aggregation, lean towards using ObjectId. However, if compatibility with certain tools or applications is your primary concern, using a string may be the way to go.

By making an informed decision based on your use case, you can optimize the performance and maintainability of your database.