Building a Notification System from Existing Data: Likes, Bookmarks, and More

When we think of adding a notification system, most of us imagine sending alerts for real-time events a user likes a post, bookmarks something, leaves a review, etc. But what if those actions already happened before the notification system was even built?

That was the exact situation I faced. Here's how I handled it.

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The Problem

In my product Rekkoku, users could already interact with posts liking them, bookmarking interesting places, or reviewing content.

But something was missing: feedback. There was no way for users to know when their content was being engaged with.

Why? because at that time, Rekkoku didn't have a notification system yet.

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Step 1: Define the Notification Model

The firts step was to create a new Notification model. I designed it with essential attributes:

•user_id : who should receive the notification

•actor_id : who triggered the action

•type : like, bookmark, review, etc.

•related_entity_id : the target post/place/etc.

•read : boolean to mark when the notification was read

This structure gave me flexibility for multiple types of events.

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Step 2: Build Routes, Controllers, and Services

I created basic API routes to interact with notifications:

•GET /notifications : retrieve all notifications for a user

•POST /notifications/mark/:id : mark one as read

•POST /notifications/mark-all-read : mark all as read

Theses APIs are fetched manually by the frontend either on page load, or on some user action like opening a notification panel.

💡Note: This is not a real-time system, there's no WebSocket or push notification involved (yet). It works more like polling, data is fetched when the user opens the notification panel.

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Hmmm...But where's the Data?

After setting up everything, I ran the project and opened the notification panel.

EMPTY

Despite knowing that users had already liked and bookmarked plenty of things, none of that data showed up.

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Step 3: Syncing Old Data

The solution? I wrote a data migration script to generate notifications based on existing likes and bookmarks.

•Loop throught all likes/bookmarks

•For each entry, check if a notification already exists (avoid duplicates)

•Insert a new notification if not found

I also added event triggers for new activities like:

•Reviewing a post

•Reviewing a place

(These will be covered in another post)

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Final Result

After running the script, I refreshed the notification view...

Tadaaa!🎉

Everything was there, neatly displayed, with the ability to mark items as read. A once empty screen was now full of meaningful activity.

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