How to check if client's contacts are using my app? - firebase

Question

I'm currently developing an app using Firebase.
My Firestore Database looks like below:
Once the user passes the Firebase authentication procedure, I'm creating a user document with a field "Phone:" which contains his phone number. Basically, everyone who is gonna using the app will be listed in the database.
And here is my challenge:
I'm using the plugin easy_contact_picker to store all the contacts of the users device to a List.
How can I find out whether the users contacts are using the app or whether they are listed in the database?
My goal is create a contact List Widget which shows me all my contacts. But those contacts which are using the app or which are listed in the database, should be highlighted or marked particularly.
Which is the best way to realize that if we consider that millions of users (to minimize computing power)are listed in the database?
Anyone has an idea?
Thanks a lot

Answer 1

First of all try to awoid giving everyone access to read all users. That is something most ppl do when handling such a problem. The do it because the query over all users won't work if you don't give the rights to read all of them.
Because of security reasons I would move the logic for checking if a user exists into callable function (not a http function!). That way you can call it inside of your app and check for a single user or multiple of them in an array. That would depend how your frontend would handle it.
Very importand would be to store all phone numbers in the absolute same format. That way you could query for them. Regardless of the number of users you could always find a specific one like here:
var citiesRef = db.collection("users");
var query = citiesRef.where("Phone", "==", "[phone]");
The numbers need to be absolutely the same without any emtpy spaces - chars and the +49 or 0049 also needs to be the same.
You could create two callable funcitons. One to check if a single user exists in your app and another where you send an array of phone numbers and you get an array back. The cloud function can use Promise.all to performe such queries in parallel so you get your responce quite fast.
I'm using a similar approach to add users in my app as admins to specific groups where you just enter the email of the user and if he is in the app he will be added. I not he get's an invitation on the email to join the App.

Answer 2

With the help of Tarik's answer, Ayrix and I came up with the following solution.
Important: Read Tarik's answer for more information.
Client: callable_compare_contacts.dart
import 'package:cloud_functions/cloud_functions.dart';
Future<List<Object>> getMembersByPhoneNumber(List<String> allPhoneNumbers) async {
HttpsCallable callable = FirebaseFunctions.instance.httpsCallable('membersByPhoneNumber');
final results = await callable.call(<String, dynamic>{'allPhoneNumbers': allPhoneNumbers});
return results.data;
}
Server: index.js
const functions = require("firebase-functions");
const admin = require("firebase-admin");
if (admin.apps.length === 0) {
admin.initializeApp({
credential: admin.credential.applicationDefault(),
});
}
exports.membersByPhoneNumber = functions.https.onCall((data, context) => {
return new Promise((resolve, reject) => {
if (!data || !data.allPhoneNumbers.length) return resolve([]);
const phoneNumbers = data.allPhoneNumbers;
// TODO: different scope? move vars for future use
const db = admin.firestore();
const collectionRef = db.collection("User");
let batches = [];
// because of wrong eslint parsing (dirty)
batches = [];
while (phoneNumbers.length) {
// firestore limits batches to 10
const batch = phoneNumbers.splice(0, 10);
// add the batch request to to a queue
batches.push(
new Promise((response) => {
collectionRef.where("Phone", "in", [...batch]).get()
.then((results) =>
response(results.docs.map(function(result) {
return result.data().Phone;
} )));
})
);
}
// response / return to client
Promise.all(batches).then(function(content) {
// console.log("content.flat()");
// console.log(content.flat());
return resolve(content.flat());
});
});
});
Note: This is our first callable/cloud function .. so Suggestions for changes are welcome.