firebase - Limit a number of documents in a subcollection in firestore rules

Question

My request is pretty simple in appearence : limiting the number of documents in a subcollection using Firestore Rules.

I already know that it is possible to know the size of an array using .size(). So, you may ask why I am not using an array to store my items ? Well, I want a "normal" user to not be able to update my parent document field but to be able to add a document to my subcollection.

I already tried to do this :

match /slots/{slot} {
      allow read: if true;
      allow write: if getRole() == 'ADMIN';

      match /participants/{participant} {
        allow read: if true;
        allow create: if get(/databases/$(database)/documents/slots/$(slot)/participants).size() < 2;
        allow delete: if participant.data.id == request.auth.uid || getRole() == 'ADMIN';
      }
    }

But get(/databases/$(database)/documents/slots/$(slot)/participants) does not work as get() should only be used for fetching a single document.

Maybe I can try something with cloud functions...

I there any solution to my problem ? Thank you for your help !

Answer 1

Waitting for answers