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Discover how to efficiently manage your MongoDB change streams to filter `update` operations while still capturing `insert` changes using Mongoose.
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How to Filter MongoDB Changes by operationType with Mongoose

When working with MongoDB, particularly in real-time applications, you might find yourself needing to listen for changes within your database. This is where MongoDB's change streams come into play. However, managing these change streams can sometimes lead to unexpected challenges and complexities.

In this guide, we will take a closer look at a specific challenge: filtering change stream data to include only update operations while still allowing for other changes like insert to be processed without any modifications. If you've faced a similar issue, read on!

The Initial Problem

Imagine you're listening to changes in your MongoDB replica set. You have already set up a change stream that retrieves the full document along with the changed fields for operationType, specifically focusing on update.

Here’s what your initial setup looked like:

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

Issue Encountered

The main issue you faced was that by filtering only for update operations, you were filtering out any insert operations that you also wanted to monitor.

Attempts to Resolve

You discovered that removing the $match filter enabled you to capture insert operations, but the $project still applied to the resulting documents.

The Solution

After some experimentation and research, the solution came to light! By adjusting the filtering approach, you can ensure that the $project stage only affects update operations while still allowing insert operations to be processed normally.

Updated Code Example

Here's how to set up your change stream to achieve this:

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

Explanation of the Code

$match Stage: Here, you match both update and insert operations. This ensures that you're receiving notifications for both operation types without excluding anything prematurely.

$project Stage with $cond: Instead of applying $project indiscriminately, you now use the $cond operator:

If operationType is update, it applies the desired modifications to the document.

If it’s an insert, it returns the full document unchanged.

Conclusion

With this method, you seamlessly filter your MongoDB change streams to only modify the output for update operations while letting insert operations pass through untouched. Now, you can effectively manage changes in your MongoDB database with precision.

Happy coding, and thank you for following along!