firebase - When are writeFields specified in Firestore requests and what replaces them?

Question

The simulator now displays an error message trying to access request.writeFields.
Before that writeFields in Firestore Security Rules did just not work in real requests.

The message states the following:

The simulator only simulates client SDK calls; request.writeFields is always null for these simulations

Does this mean that writeFields are only specified in HTTP requests?

The documentation only states this:

writeFields: List of fields being written in a write request.

A problem that arises from this

I am searching for something that replaces this property because it is "always null".
request.resource.data in update also contains fields that are not in the requests, but already in the document to my knowledge.

Example

// Existing document:

  document:
    - name: "Peter"
    - age: 52
    - profession: "Baker"

// Update call:

  document:
    - age: 53

// request.resource.data in allow update contains the following:

  document:
    - name: "Peter"
    - age: 53
    - profession: "Baker"

But I only want age.

Answer 1

Waitting for answers