Q:

Cloud Functions: How to copy Firestore Collection to a new document?

I'd like to make a copy of a collection in Firestore upon an event using Cloud Functions
I already have this code that iterates over the collection and copies each document
const firestore = admin.firestore()
firestore.collection("products").get().then(query => {
  query.forEach(function(doc) {
    var promise = firestore.collection(uid).doc(doc.data().barcode).set(doc.data());
  });
});

is there a shorter version? to just copy the whole collection at once?

A:

Currently, no. Looping through each document using Cloud Functions and then setting a new document to a different collection with the specified data is the only way to do this. Perhaps this would make a good feature request. 
How many documents are we talking about? For something like 10,000 it should only take a few minutes, tops.

A:

I wrote a small nodejs snippet for this.
const firebaseAdmin = require('firebase-admin');
const serviceAccount = '../../firebase-service-account-key.json';
const firebaseUrl = 'https://my-app.firebaseio.com';

firebaseAdmin.initializeApp({
    credential: firebaseAdmin.credential.cert(require(serviceAccount)),
    databaseURL: firebaseUrl
});
const firestore = firebaseAdmin.firestore();

async function copyCollection(srcCollectionName, destCollectionName) {
    const documents = await firestore.collection(srcCollectionName).get();
    let writeBatch = firebaseAdmin.firestore().batch();
    const destCollection = firestore.collection(destCollectionName);
    let i = 0;
    for (const doc of documents.docs) {
        writeBatch.set(destCollection.doc(doc.id), doc.data());
        i++;
        if (i > 400) {  // write batch only allows maximum 500 writes per batch
            i = 0;
            console.log('Intermediate committing of batch operation');
            await writeBatch.commit();
            writeBatch = firebaseAdmin.firestore().batch();
        }
    }
    if (i > 0) {
        console.log('Firebase batch operation completed. Doing final committing of batch operation.');
        await writeBatch.commit();
    } else {
        console.log('Firebase batch operation completed.');
    }
}

copyCollection('customers', 'customers_backup').then(() => console.log('copy complete')).catch(error => console.log('copy failed. ' + error));

