Firebase cloud functions not waiting for forEach to complete before jumping to the next then - google-cloud-firestore

Question

Been trying to copy subcollections of a collection into another collection. The code below is aimed at that, but jumps from the first then and logs out "Done" without logging out anything before.
So the question is what is not correct here?
exports = module.exports = functions.https.onRequest(async (req, res) => {
let db = admin.firestore();
try {
await db.collection("users").get().then((query) => {
return query.forEach(async (doc) => {
console.log("Here"); //This doesn't print
const polCollection = await db.collection("users").doc(doc.id).collection("xyz").get();
if (polCollection.docs.length > 0) { //This checks if any subcollections
for (const x of polCollection.docs) { //This copies them into a doc in the copy collection
db.collection("CopyUsers")
.doc(doc.id)
.set({ x : x.data() }, { merge: true });
}
}
});
})
.then(() => {
console.log("Done"); //This is the only thing that prints in the console
res.end();
})
.catch((e) => {
console.log("e", e);
res.end();
});
} catch (error) {
console.log("error", error);
res.end();
}
});
After the suggestion below, it now looks as follows:
exports = module.exports = functions.runWith(runtimeOpts).https.onRequest(async (req, res) => {
const promises = [];
let count = 0;
let size = 0;
return await admin
.firestore()
.collection("testUsers")
.get()
.then((query) => {
console.log("query length:", query.size); //prints x of users
size = query.size;
query.forEach(async (doc) => {
const promise = async () => {
console.log("Here", doc.id); //This doesn't print
await admin
.firestore()
.collection("testUsers")
.doc(doc.id)
.collection("xyz")
.get()
.then(async (polCollection) => {
if (polCollection.docs.length > 0) {
for (const x of polCollection.docs) {
return await admin
.firestore()
.collection("testBackUpUsers")
.doc(doc.id)
.set(
{ xyz: x.data() },
{ merge: true }
);
}
} else {
return;
}
})
.catch((e) => console.log("error from then after get xyz", e));
};
count++;
return promises.push(promise);
});
return promises;
})
.then(async (promises) => {
if (size <= count) {
console.log("running return Promise.all(promises)", promises.length); //prints number of promises = users
return Promise.all(promises);
}
})
.catch((e) => console.log("err from the last catch", e));
});
Any thoughts?

Answer 1

Unfortunately the forEach iterator does not support async/await. Even if you write an await inside it will just go trough it without waiting on the execution.
I would recommend to use Promise.all. That would also execute the code in parallel and would finish faster.
If you would only change data you could also use a batch change but in your example you first get the data and then change it.
Here is an example how you could write your code:
exports = module.exports = functions.https.onRequest(async (req, res) => {
let db = admin.firestore();
const promises = [];
try {
const query = await db.collection("users").get();
query.forEach((doc) => {
console.log("doc", doc);
const promise = async () => {
console.log("Here", doc.id); //This doesn't print
const polCollection = await db
.collection("users")
.doc(doc.id)
.collection("xyz")
.get();
if (polCollection.docs.length > 0) {
//This checks if any subcollections
for (const x of polCollection.docs) {
//This copies them into a doc in the copy collection
await db
.collection("CopyUsers")
.doc(doc.id)
.set({ x: x.data() }, { merge: true });
}
}
};
promises.push(promise);
});
console.log("promises", promises);
await Promise.all(promises);
console.log("Done");
res.end();
} catch (error) {
console.log("error", error);
res.end();
}
});