How to get a count of number of documents in a collection with Cloud Firestore

Question

You currently have 3 options:

Option 1: Client side

This is basically the approach you mentioned. Select all from collection and count on the client side. This works well enough for small datasets but obviously doesn't work if the dataset is larger.

Option 2: Write-time best-effort

With this approach, you can use Cloud Functions to update a counter for each addition and deletion from the collection.

This works well for any dataset size, as long as additions/deletions only occur at the rate less than or equal to 1 per second. This gives you a single document to read to give you the almost current count immediately.

If need need to exceed 1 per second, you need to implement distributed counters per our documentation.

Option 3: Write-time exact

Rather than using Cloud Functions, in your client you can update the counter at the same time as you add or delete a document. This means the counter will also be current, but you'll need to make sure to include this logic anywhere you add or delete documents.

Like option 2, you'll need to implement distributed counters if you want to exceed per second

Aggregations are the way to go (firebase functions looks like the recommended way to update these aggregations as client side exposes info to the user you may not want exposed) https://firebase.google.com/docs/firestore/solutions/aggregation

Another way (NOT recommended) which is not good for large lists and involves downloading the whole list: res.size like this example:

   db.collection("logs")
      .get()
      .then((res) => console.log(res.size));

If you use AngulareFire2, you can do (assuming private afs: AngularFirestore is injected in your constructor):

this.afs.collection(myCollection).valueChanges().subscribe( values => console.log(values.length));

Here, values is an array of all items in myCollection. You don't need metadata so you can use valueChanges() method directly.

Be careful counting number of documents for large collections with a cloud function. It is a little bit complex with firestore database if you want to have a precalculated counter for every collection.

Code like this doesn't work in this case:

export const customerCounterListener = 
    functions.firestore.document('customers/{customerId}')
    .onWrite((change, context) => {

    // on create
    if (!change.before.exists && change.after.exists) {
        return firestore
                 .collection('metadatas')
                 .doc('customers')
                 .get()
                 .then(docSnap =>
                     docSnap.ref.set({
                         count: docSnap.data().count + 1
                     }))
    // on delete
    } else if (change.before.exists && !change.after.exists) {
        return firestore
                 .collection('metadatas')
                 .doc('customers')
                 .get()
                 .then(docSnap =>
                     docSnap.ref.set({
                         count: docSnap.data().count - 1
                     }))
    }

    return null;
});

The reason is because every cloud firestore trigger has to be idempotent, as firestore documentation say: https://firebase.google.com/docs/functions/firestore-events#limitations_and_guarantees

Solution

So, in order to prevent multiple executions of your code, you need to manage with events and transactions. This is my particular way to handle large collection counters:

const executeOnce = (change, context, task) => {
    const eventRef = firestore.collection('events').doc(context.eventId);

    return firestore.runTransaction(t =>
        t
         .get(eventRef)
         .then(docSnap => (docSnap.exists ? null : task(t)))
         .then(() => t.set(eventRef, { processed: true }))
    );
};

const documentCounter = collectionName => (change, context) =>
    executeOnce(change, context, t => {
        // on create
        if (!change.before.exists && change.after.exists) {
            return t
                    .get(firestore.collection('metadatas')
                    .doc(collectionName))
                    .then(docSnap =>
                        t.set(docSnap.ref, {
                            count: ((docSnap.data() && docSnap.data().count) || 0) + 1
                        }));
        // on delete
        } else if (change.before.exists && !change.after.exists) {
            return t
                     .get(firestore.collection('metadatas')
                     .doc(collectionName))
                     .then(docSnap =>
                        t.set(docSnap.ref, {
                            count: docSnap.data().count - 1
                        }));
        }

        return null;
    });

Use cases here:

/**
 * Count documents in articles collection.
 */
exports.articlesCounter = functions.firestore
    .document('articles/{id}')
    .onWrite(documentCounter('articles'));

/**
 * Count documents in customers collection.
 */
exports.customersCounter = functions.firestore
    .document('customers/{id}')
    .onWrite(documentCounter('customers'));

As you can see, the key to prevent multiple execution is the property called eventId in the context object. If the function has been handled many times for the same event, the event id will be the same in all cases. Unfortunately, you must have "events" collection in your database.

Please check below answer I found on another thread. Your count should be atomic. Its required to use FieldValue.increment() function in such case.

https://stackoverflow.com/a/49407570/3337028