Update Cloud Firestore Collection Based on "Master Collection" - swift

Question

I am creating an iOS application to help users who collect figurines keep track of their figurines and see realtime price information.
I have two collections:
The "master" collection
This is essentially a catalog of thousands of figurines users can select to add to their collection. It has information about each item, such as Artist, Original Price, Barcode, Year, Comments, etc.
The "user" collection
This contains documents for each user. Each of these "user" documents has a sub-collection with documents for each figurine they have added to their collection.
Currently, if a user wants to add a new item to their personal collection, they go to the "master" collection UITableView and swipe to add an item. The item document is basically copied from the "master" collection and a new document in the user sub-collection is created. They can then go to a UITableView that displays the items they have added. They have the option to add their own information about the item, such as comments, date bought, date sold, price bought, etc.
My question: How can I sync information between the document in the user collection/sub-collection and the document in the main collection? Say the "Market Price" for an item has gone up, so I update the main collection database. How can I get this price change to display in all users' collections?
I assume by using Cloud Functions, but I wanted to post here in case there was an easier method.

Answer 1

This is definitely a good use for Cloud Functions. Create an onUpdate trigger for your /users{uid}/figurines/{figurineId} collection and then use that to copy the updated data to the master.
You can either copy specific fields or you can check for differences by examining event.data.data() and event.data.previous.data()
const admin = require('firebase-admin');
const functions = require('firebase-functions');
admin.initializeApp(functions.config().firebase);
const db = admin.firestore();
exports.updateMaster = functions.firestore
.document('users/{uid}/figurines/{figurineId}')
.onUpdate(event => {
var newData = event.data.data();
var oldData = event.data.previous.data();
var updateData = {};
if (oldData.price != newData.price) {updateData.price = newData.price}
// Add any more changes that you may want to compare / update below...
// Then...
return db
.collection('master')
.doc(event.params.figurineId)
.update(updateData).then(response => {
console.log('We updated figurine', event.params.figurineId);
}).catch(err => {
console.error('There was an error', err);
});
});