Trouble reading data in Firebase Cloud Function - firebase

Question

Trying to read a pushToken from a given user in the users collection (after an update operation on another collection) returns undefined
exports.addDenuncia = functions.firestore
.document('Denuncias/{denunciaID}')
.onWrite((snap, context) => {
const doc = snap.after.data()
const classificadoId = doc.cid
const idTo = doc.peerId
db.collection('Classificados').doc(classificadoId)
.update({
aprovado: false
})
.then(r => {
getToken(idTo).then(token => {
// sendMsg...
})
}).catch(updateErr => {
console.log("updateErr: " + updateErr)
})
async function getToken(id) {
let response = "getTokenResponse"
console.log("id in getToken: " + id)
return db.collection('users').doc(id).get()
.then(user => {
console.log("user in getToken: " + user.data())
response = user.data().pushToken
})
.catch(e => {
console.log("error get userToken: " + e)
response = e
});
return response
}
return null
});
And this is from the FB console log:
-1:43:33.906 AM Function execution started
-1:43:36.799 AM Function execution took 2894 ms, finished with status: 'ok'
-1:43:43.797 AM id in getToken: Fm1RwJaVfmZoSgNEFHq4sbBgoEh1
-1:43:49.196 AM user in getToken: undefined
-1:43:49.196 AM error get userToken: TypeError: Cannot read property 'pushToken' of undefined
-1:43:49.196 AM returned token: undefined
And we can see in this screenshot from the db that the doc does exist:
Hope someone can point me to what I'm doing wrong here.
added screenshot of second example of #Renaud as deployed:

Answer 1

As Doug wrote in his comment, you need to "return a promise from the top level function that resolves when all the async work is complete". He also explains that very well in the official video series: https://firebase.google.com/docs/functions/video-series/ (in particular the 3 videos titled "Learn JavaScript Promises"). You should definitely watch them, highly recommended!
So, the following modifications to your code should work (untested):
exports.addDenuncia = functions.firestore
.document('Denuncias/{denunciaID}')
.onWrite(async (snap, context) => { // <- note the async keyword
try {
const doc = snap.after.data()
const classificadoId = doc.cid
const idTo = doc.peerId
await db.collection('Classificados').doc(classificadoId)
.update({
aprovado: false
});
const userToSnapshot = await db.collection('users').doc(idTo).get();
const token = userToSnapshot.data().pushToken;
await sendMsg(token); // <- Here you should take extra care to correctly deal with the asynchronous character of the sendMsg operation
return null; // <-- This return is key, in order to indicate to the Cloud Function platform that all the asynchronous work is done
} catch (error) {
console.log(error);
return null;
}
});
Since you use an async function in your code, I've used the async/await syntax but we could very well write it by chaining the promises with the then() method, as shown below.
Also, I am not sure, in your case, that it adds any value to put the code that gets the token in a function (unless you want to call it from other Cloud Functions but then you should move it out of the addDenuncia Cloud Function). That's why it has been replaced by two lines of code within the main try block.
Version with chaining promises via the then() method
In this version we chain the different promises returned by the asynchronous methods with the then() method. Compared to the async/await version above, it shows very clearly what means "to return a promise from the top level function that resolves when all the asynchronous work is complete".
exports.addDenuncia = functions.firestore
.document('Denuncias/{denunciaID}')
.onWrite((snap, context) => { // <- no more async keyword
const doc = snap.after.data()
const classificadoId = doc.cid
const idTo = doc.peerId
return db.collection('Classificados').doc(classificadoId) // <- we return a promise from the top level function
.update({
aprovado: false
})
.then(() => {
return db.collection('users').doc(idTo).get();
})
.then(userToSnapshot => {
if {!userToSnapshot.exists) {
throw new Error('No document for the idTo user');
}
const token = userToSnapshot.data().pushToken;
return sendMsg(token); // Again, here we make the assumption that sendMsg is an asynchronous function
})
.catch(error => {
console.log(error);
return null;
})
});