Firestore Security Rules - How can I check that a field is/isn't being modified?

Question

For the life of me, I cannot understand why the following is resulting in a false for allowing writes. Assume my users collection is empty to start

Answer 1

Found this rule to work quite well:

function propChanged(key) {
  // Prop changed if key in req but not res, or if key req and res have same value
  return (
    (key in request.resource.data) && !(key in resource.data)
  ) || (
    (key in request.resource.data) && request.resource.data[key] != resource.data[key]
  );
}