firebase - Firestore Security Rules - allow read subcollection based on its parent document data

Question

I am implementing a chat app with Firestore, and I have a document for each chatroom in a collection "chats/" which have a subcollection "messages/" with all the messages in the room.

For accessing this subcollection, I need to check that the user is allowed to read it, checking that he is in the array of members which is in the parent document.

/chats
    /chatId
        /messages (subcollection)
            /messageId
        - members array (field)

Currently, I am doing this:

function isSignedIn() {
   return request.auth.uid != null;
}

match /chats/{chatId} {
    function isUserInChatRoom() {
      let userId = request.auth.uid;
      // A user is in the chatroom if he is in its members list
      return userId in resource.data.members;
    }
      
    // Only signed users which are in the chatroom can read its data
    allow read: if isSignedIn() && isUserInChatRoom(); 
       
    // The client side cannot modify the chats collection
    allow write, update, delete: if false;
      
    match /messages/{document=**} {
      // Only signed users which are in the chatroom can read its messages
      allow read: if isSignedIn() && isUserInChatRoom();
    }
}

But, it seems that isUserInChatRoom() doesnt work in the messages subcollection.

How can I achieve this?

Answer 1

I am not really into security rules syntax but you can try this (based on the docs)

match /chats/{chatId} {
  function isUserInChatRoom() {
    let userId = request.auth.uid;
    // A user is in the chatroom if he is in its members list
    return userId in resource.data.members;
  }

  function getChatRoomData() {
     return get(/databases/$(database)/documents/chats/$(chatId)).data;
  }
      
  // Only signed users which are in the chatroom can read its data
  allow read: if isSignedIn() && isUserInChatRoom(); 
       
  // The client side cannot modify the chats collection
  allow write, update, delete: if false;
      
  match /messages/{document=**} {
    function canReadMessages() {
      let userId = request.auth.uid;
      // A user can read the chatroom messages if he is a member of the chatroom
      return userId in getChatRoomData().members;
    }
        
    // Only signed users which are in the chatroom can read its messages
    allow read: if isSignedIn() && isUserInChatRoom();
  }
}