import * as admin from 'firebase-admin'
import * as functions from 'firebase-functions'

// As an admin, the app has read and write access to all data,
// regardless of Security Rules
const db = admin.firestore()
const regionId = 'asia-southeast1'

/**
 * Firestore trigger that update documents in another collection
 * that consist of the product id when the product is updated
 */
export const dbTriggerToUpdateAllDocuments = functions
  .region(regionId)
  .firestore.document(`products/{productId}`) // the document to watch
  .onUpdate(async (snap, context) => {
    // watch for "update" event
    try {
      const payload = snap.after.data() // data from the product document
      const id = context.params.productId
      await db
        .collection(`bookingHistory`) // documents within the collection to update
        .where('productId', '==', id) // get only documents that have their productId field matching the id
        .get()
        .then((querySnapshot) => {
          querySnapshot.forEach((doc) => {
            // the "doc" object come with an id property that we can use to update specific bookings
            db.doc(`bookingHistory/${doc.id}`).update({
              ...payload,
            })
          })
        })
    } catch (error) {
      console.error('Failed to update user bookings:', error.message)
    }
  })

Introduction

This code snippet can be customized to use a different collection as a source and update multiple documents of whatever collection you want.

I will explain the code snippet and its usage using the following scenario:

Example usage

Let's said that you have a few users, who had booked "ABC Training" on your website. "ABC Training" data came from a product document in the "products" collection, and the new booking data is stored in a collection called "bookingHistory".

Your business admin recently decided to updated the trainers who will be teaching "ABC Training" course.

Now, the "ABC Training" data in your products document have the new data entered by the admin.

But your users still have stale data (old data), where originally, the trainer was set to Van in their "bookingHistory" collection, as seen in the picture below.

In order to update all of the user bookings for "ABC Training", you need to write a cloud function to watch for an "update" event on the "products" collection.