Pre-update hook. This annotation marks a callback that is triggered when a user performs a "update" action.
This hook will be triggered after all security checks have been run, but before the datastore
has been committed.
The invoked function takes a RequestScope as parameter.
Field name on which the annotated method is only triggered if that field is modified.
If value is empty string, then trigger once when the object is updated.
If value is "*", then trigger for all field modifications.