Allow update on single field in firestore

Question

I want to give a user the right to update a document. But ONLY if the user updates one specific field of this document. All other fields shouldn't be changed by this user.

Is this possible in firestore?

I tried something like this:

function isUpdateToOpenField(attr) {
    return attr == get(/databases/$(database)/documents/stores/$(store)).data.open;
}

allow update: if isUpdateToOpenField(request.resource.data);

But I don't know how to compare if the update corresponds to the right field.

Answer 1

Update: writeFields is no longer supported by Firestore and its functionality will eventually be removed. See this post.

Check out the writeFields variable for security rules:

allow update: if ((request.writeFields.size() == 1) && ('open' in request.writeFields));