[web2py] how to change the value of a field that is writable=False for crud.update form

[selecta]

I want to change the value of a field that is writable=False of a form
that was created with crud.update

def edit():
    table, record = getTableRecord(request)
    if record:
        #this will show the right content but will not be stored in db
        record.created_on = request.now
        record.edited_by = auth.user.id
    #this does not have any effect
    table.created_on.default = request.now
    table.edited_by.default = auth.user.id

    form = crud.update(table, record, onaccept=crud.archive, deletable
= False)

    #this also does not have any effect
    form.vars.created_on = request.now
    form.vars.edited_by = auth.user.id

    return dict(form = form)