Firebase: How to prevent writing a specific field in firestore?

Question

The ruleset protects the user's entry like follows:

• only him or an admin can read his data
• only an admin can update a user's permissions
• a user can update all his data, unless he sets the permissions object

The rule looks like:

match /users/{userId} {
  allow read: if isCurrentUser(userId) || isAdmin();
  allow write: if (isCurrentUser(userId) && !isModifyingPermissions()) || isAdmin();

  function isModifyingPermissions(){
    return request.resource.data['permissions'] != null;
  }
}

I'm stuck with the isModifiyingPermissions() function. It properly refuses a write in case the request has a value for the permissions property. However, the rule crashes if no permissions property is provided, stating the following:

Error: simulator.rules line [19], column [15]. Property permissions is undefined on object.

How can one write "check presence of a property on request resource" ?

Answer 1

Ok, here's the solution:

  function isModifyingPermissions(){
    return request.resource.data.keys().hasAny(["permissions"]);
  }