Firestore security rules get field/id of reference

Question

I have two collections - tenancies and users.

A tenancy doc has a field called "landlordID" and is of type REFERENCE (not String).

Now in my Firestore Security Rules I want to allow a tenancy to be updated ONLY IF the landlordID field of that tenancy matches with the uid of the user making the request, namely request.auth.uid.

Read it as " allow a tenancy document to be updated if the user making the user is authenticated, hence request.auth.uid != null, and the landlordID field's ID should be equal to that of the request.auth.uid.

Hence the code should me something like this:

    service cloud.firestore {

      match /databases/{database}/documents {

        match /tenancies/{tenancyID}{

            allow update: if request.auth.uid != null && 
                        request.auth.uid == get(resource.data.landlordID).id
    }

}

I have also tried get(/databases/$(database)/documents/users/$(resource.data.landlordID)).data.id

Supporting screenshot of my database

This should be very simple but get() simply does not work. Firebase Docs, scroll to "Access other documents" was not helpful at all for my situation and I am not sure how to get it working.

It would be a shame if references can't be used like this as they are just like any other field of a document.

Answer 1

Here is a function I made that works for me. I guess you have a user collection with users having the same id as their auth.uid

    function isUserRef(field) {
      return field in resource.data
        && resource.data[field] == /databases/$(database)/documents/users/$(request.auth.uid)
    }

Adjusting to your use case you'd call the function so: isUserRef('landlordID') although the ID at the end of it is a bit misleading as this field is in fact a reference.