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

The solution of Tom Bailey (https://stackoverflow.com/a/48177722/5727205) did look promising.

But in my case I needed to prevent a field from being edited, and could have the case, that the field simply does not exist on the existing data. Thereby I added a check if the field exists.

This solution does check two checks:

1. If the field is not in request and non in the existing data (equals field not modified)
2. or request and existing data are the same (equals field not modified)

 function isNotUpdatingField(fieldName) {
   return
     ( !(fieldName in request.resource.data) && !(fieldName in resource.data) ) || 
     request.resource.data[fieldName] == resource.data[fieldName];
}