How to write an authentication rule to verify captcha on Google Firebase

Question

I want to make a simple Form to collect user emails on my website deployed on Firebase. I want to prevent bots thus I enabled an 'App Check' with ReCaptchaV3. User must be able to post only one submission and since my website doesn't have registration I implemented an anonymous authentication so that ID is automatically set for each user.

How do I check that user is authenticated and captcha is passed in Firestore rules?

Here is code for a function to write a submission

export const writeUserData = async (email) => {
    const db = getFirestore();
    const uid = auth.currentUser?.uid
    const docRef = await addDoc(collection(db, `newsletter/${uid}/entries`), {
        email: email
    });
    console.log(docRef);
}

Here is Firestore ruleset

rules_version = '2';

service cloud.firestore {
    match /databases/{database}/documents {
        allow read: if false;

        match /newsletter/{userId}/entries {
          allow write: if request.auth != null;
        }
  }
}

What do I get:

Uncaught (in promise) FirebaseError: Missing or insufficient permissions.

Answer 1

I end up with the following rule set

rules_version = '2';

service cloud.firestore {
  match /databases/{database}/documents {
    match /newsletter/{userId}/entries/{document=**} {
        allow write: if request.auth != null && request.auth.uid == userId;
      }
  }
}

It works only if I add {document=**} to the collection I want edit