How do you force a Firestore client app to maintain a correct document count for a collection?

Question

Firestore doesn\'t have a way to query for the size of a collection, so if a client app needs to know this value, it requires some maintenance of a document in another colle

Answer 1

The suggested solution would still fail, as you could just add additional documents to the batch write.

But you could just add the following to the message collection rule:

&& get(/databases/$(database)/documents/messages-stats/data).data.messageId == request.resource.id;

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {

    match /messages/{id} {
      allow read;
      allow create: if
        getAfter(/databases/$(database)/documents/messages-stats/data).data.count ==
             get(/databases/$(database)/documents/messages-stats/data).data.count + 1 
             && get(/databases/$(database)/documents/messages-stats/data).data.messageId == request.resource.id;
      allow delete: if
        getAfter(/databases/$(database)/documents/messages-stats/data).data.count ==
             get(/databases/$(database)/documents/messages-stats/data).data.count - 1
             && get(/databases/$(database)/documents/messages-stats/data).data.messageId == request.resource.id;
    }

    match /messages-stats/data {
      allow read;
      allow update: if (
        request.resource.data.count == resource.data.count + 1 &&
        existsAfter(/databases/$(database)/documents/messages/$(request.resource.data.messageId)) &&
           ! exists(/databases/$(database)/documents/messages/$(request.resource.data.messageId))
      ) || (
        request.resource.data.count == resource.data.count - 1 &&
        ! existsAfter(/databases/$(database)/documents/messages/$(request.resource.data.messageId)) &&
               exists(/databases/$(database)/documents/messages/$(request.resource.data.messageId))
      );
    }

  }
}