Making automated notifications with Firebase Cloud Functions, Messaging,Firestore - firebase

Question

I've been trying to push notifications with .onUpdate() trigger but It doesn't work. I'm not sure what is wrong since anything I find on docs is useless pretty much and it's my first time working with Node.js.
I want to notify the user (with Firebase Messaging) when any product gets updated (in Firebase Realtime Database) using Firebase Cloud Functions, which is after submitting an order, and the requirement is that the product stock is <= 5.
Structure of the collection is like this:
products (collection) -> {productID} (document) -> attributes: {name, barcode, price, stock, sold}
//import firebase
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.sendNotification = functions.database.ref('/products/{product}')
.onUpdate((change, context) => {
const prodName = context.data.child('name');
const numProd = context.data.child('stock');
if(numProd<=5){
const payload = {
notification: {
title: 'Low stock!',
body: `Product ${prodName} is running out.`
}
}
const registrationToken = 'token';
return admin.messaging().sendToDevice(registrationToken,payload)
.then(function(response){
console.log('Notification sent successfully:',response);
return 1;
})
.catch(function(error){
console.log('Notification sent failed:',error);
});
}
});

Answer 1

Apparently you are mixing up the two Firebase's database services: Firestore and the Realtime Database.
As a matter of fact, you indicate that your data is organised in collections ("Structure of the collection is like this: products (collection) -> {productID} (document)") which means that you are using Firestore (Realtime Database doesn't have collections).
But your background trigger is corresponding to a Realtime Database trigger, see https://firebase.google.com/docs/functions/database-events.
If the assumption that you are mixing up the two database services is right, you need to use a background trigger for Firestore, see https://firebase.google.com/docs/functions/firestore-events, in particular the onUpdate() one, as follows:
exports.updateUser = functions.firestore
.document('/products/{productId}')
.onUpdate((change, context) => {
// Get an object representing the document
const newValue = change.after.data();
const prodName = newValue.name;
const numProd = newValue.stock;
// ...
});
Note that it seems that you are not handling correctly the case when numProd > 5. You may throw an error or just do return null;
It is a also a good idea to watch the 3 videos about "JavaScript Promises" from the Firebase video series: https://firebase.google.com/docs/functions/video-series/.