firebase - Firestore security rules: How to validate that a field is undefined?

Question

When a user signs up and they initialise their data in firestore, I want to validate that they aren't attempting to set their role (i.e. so they're not setting it to 'admin' for example).

I tried to write this:

match /users/{userId} {
  allow create: if (signedInAs(userId) && !request.resource.data.role) || isAdmin();
  ...

...but I just see "Property role is undefined on object."

Is there a way to do this safely? Or does this mean I should always be initialising expected fields, even if it's just to the empty string? That doesn't seem to quite fit with the philosophy of NoSQL?

Answer 1

Waitting for answers