Firestore rules access to parent document - firebase

Question

So i'm making an app with a friends system and trying to set up rules for firebase to handle reads & writes if the users is friends or not.
I'm very stuck at a particular call that i just don't have any idea on how to make.
My firestore is structured as follows:
users/userUID/places/documentsofplaces
each userdocument have some fields of the usual information, name, username, etc. and an array of friendsUID.
I have managed to get the first part down, that a user can only read and write if it's UID matches the documentUID, and looking in the friendslist a friend can only read but not write.
The next part, in the places collection, just throws me off, how can i get the parent document and compare the userUID to a UID in the friendslist?
This is what i have so far:
rules_version = '2';
service cloud.firestore {
match /databases/{database}/documents {
match /users/{userId} {
// Allow write and read if user, and read if friend
allow write: if isUser(userId);
allow read: if isUser(userId) || isFriend();
function isUser(userId) {
return (request.auth.uid == userId);
}
function isFriend() {
return (request.auth.uid in resource.data.friendsList);
}
}
match /users/{userId}/places/{documents} {
allow write: if isUser(userId);
allow read: if isUser(userId) || isFriend(userId);
function isUser(userId) {
return (request.auth.uid == userId);
}
function isFriend(userId) {
return (request.auth.uid in get(/databases/$(database)/documents/users/userId.resource.data.friendsList));
}
}
}
}
Any help is greatly appreciated!

Answer 1

Your document get() should look more like this:
get(/databases/$(database)/documents/users/$(userId)).data.friendsList
You have to use variables with $(var) notation inside the document path. get() returns a Resoruce object with a data property. I suggest reading over the documentation for accessing other documents for more details.