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

Increment a counter using admin.firestore.FieldValue.increment:

exports.onInstanceCreate = functions.firestore.document('projects/{projectId}/instances/{instanceId}')
  .onCreate((snap, context) =>
    db.collection('projects').doc(context.params.projectId).update({
      instanceCount: admin.firestore.FieldValue.increment(1),
    })
  );

exports.onInstanceDelete = functions.firestore.document('projects/{projectId}/instances/{instanceId}')
  .onDelete((snap, context) =>
    db.collection('projects').doc(context.params.projectId).update({
      instanceCount: admin.firestore.FieldValue.increment(-1),
    })
  );

In this example we increment an instanceCount field in the project each time a document is added to the instances sub collection. If the field doesn't exist yet it will be created and incremented to 1.

The incrementation is transactional internally but you should use a distributed counter if you need to increment more frequently than every 1 second.

It's often preferable to implement onCreate and onDelete rather than onWrite as you will call onWrite for updates which means you are spending more money on unnecessary function invocations (if you update the docs in your collection).