Cloud Firestore collection count

Question

Is it possible to count how many items a collection has using the new Firebase database, Cloud Firestore?

If so, how do I do that?

Answer 1

As far as I know there is no build-in solution for this and it is only possible in the node sdk right now. If you have a

db.collection('someCollection')

you can use

.select([fields])

to define which field you want to select. If you do an empty select() you will just get an array of document references.

example:

db.collection('someCollection').select().get().then( (snapshot) => console.log(snapshot.docs.length) );

This solution is only a optimization for the worst case of downloading all documents and does not scale on large collections!

Also have a look at this:
How to get a count of number of documents in a collection with Cloud Firestore

Answer 2

In 2020 this is still not available in the Firebase SDK however it is available in Firebase Extensions (Beta) however it's pretty complex to setup and use...

A reasonable approach

Helpers... (create/delete seems redundant but is cheaper than onUpdate)

export const onCreateCounter = () => async (
  change,
  context
) => {
  const collectionPath = change.ref.parent.path;
  const statsDoc = db.doc("counters/" + collectionPath);
  const countDoc = {};
  countDoc["count"] = admin.firestore.FieldValue.increment(1);
  await statsDoc.set(countDoc, { merge: true });
};

export const onDeleteCounter = () => async (
  change,
  context
) => {
  const collectionPath = change.ref.parent.path;
  const statsDoc = db.doc("counters/" + collectionPath);
  const countDoc = {};
  countDoc["count"] = admin.firestore.FieldValue.increment(-1);
  await statsDoc.set(countDoc, { merge: true });
};

export interface CounterPath {
  watch: string;
  name: string;
}

Exported Firestore hooks

export const Counters: CounterPath[] = [
  {
    name: "count_buildings",
    watch: "buildings/{id2}"
  },
  {
    name: "count_buildings_subcollections",
    watch: "buildings/{id2}/{id3}/{id4}"
  }
];


Counters.forEach(item => {
  exports[item.name + '_create'] = functions.firestore
    .document(item.watch)
    .onCreate(onCreateCounter());

  exports[item.name + '_delete'] = functions.firestore
    .document(item.watch)
    .onDelete(onDeleteCounter());
});

In action

The building root collection and all sub collections will be tracked.

Here under the /counters/ root path

Now collection counts will update automatically and eventually! If you need a count, just use the collection path and prefix it with counters.

const collectionPath = 'buildings/138faicnjasjoa89/buildingContacts';
const collectionCount = await db
  .doc('counters/' + collectionPath)
  .get()
  .then(snap => snap.get('count'));

Limitations

As this approach uses a single database and document, it is limited to the Firestore constraint of 1 Update per Second for each counter. It will be eventually consistent, but in cases where large amounts of documents are added/removed the counter will lag behind the actual collection count.

Answer 3

I agree with @Matthew, it will cost a lot if you perform such query.

[ADVICE FOR DEVELOPERS BEFORE STARTING THEIR PROJECTS]

Since we have foreseen this situation at the beginning, we can actually make a collection namely counters with a document to store all the counters in a field with type number.

For example:

For each CRUD operation on the collection, update the counter document:

1. When you create a new collection/subcollection: (+1 in the counter) [1 write operation]
2. When you delete a collection/subcollection: (-1 in the counter) [1 write operation]
3. When you update an existing collection/subcollection, do nothing on the counter document: (0)
4. When you read an existing collection/subcollection, do nothing on the counter document: (0)

Next time, when you want to get the number of collection, you just need to query/point to the document field. [1 read operation]

In addition, you can store the collections name in an array, but this will be tricky, the condition of array in firebase is shown as below:

// we send this
['a', 'b', 'c', 'd', 'e']
// Firebase stores this
{0: 'a', 1: 'b', 2: 'c', 3: 'd', 4: 'e'}

// since the keys are numeric and sequential,
// if we query the data, we get this
['a', 'b', 'c', 'd', 'e']

// however, if we then delete a, b, and d,
// they are no longer mostly sequential, so
// we do not get back an array
{2: 'c', 4: 'e'}

So, if you are not going to delete the collection , you can actually use array to store list of collections name instead of querying all the collection every time.

Hope it helps!

Answer 4

A workaround is to:

write a counter in a firebase doc, which you increment within a transaction everytime you create a new entry

You store the count in a field of your new entry (i.e: position: 4).

Then you create an index on that field (position DESC).

You can do a skip+limit with a query.Where("position", "<" x).OrderBy("position", DESC)

Hope this helps!

Answer 5

This uses counting to create numeric unique ID. In my use, I will not be decrementing ever, even when the document that the ID is needed for is deleted.

Upon a collection creation that needs unique numeric value

1. Designate a collection appData with one document, set with .doc id only
2. Set uniqueNumericIDAmount to 0 in the firebase firestore console
3. Use doc.data().uniqueNumericIDAmount + 1 as the unique numeric id
4. Update appData collection uniqueNumericIDAmount with firebase.firestore.FieldValue.increment(1)

firebase
    .firestore()
    .collection("appData")
    .doc("only")
    .get()
    .then(doc => {
        var foo = doc.data();
        foo.id = doc.id;

        // your collection that needs a unique ID
        firebase
            .firestore()
            .collection("uniqueNumericIDs")
            .doc(user.uid)// user id in my case
            .set({// I use this in login, so this document doesn't
                  // exist yet, otherwise use update instead of set
                phone: this.state.phone,// whatever else you need
                uniqueNumericID: foo.uniqueNumericIDAmount + 1
            })
            .then(() => {

                // upon success of new ID, increment uniqueNumericIDAmount
                firebase
                    .firestore()
                    .collection("appData")
                    .doc("only")
                    .update({
                        uniqueNumericIDAmount: firebase.firestore.FieldValue.increment(
                            1
                        )
                    })
                    .catch(err => {
                        console.log(err);
                    });
            })
            .catch(err => {
                console.log(err);
            });
    });