(Quick Reference)
                getPersistentValue
Purpose
Retrieve the original value of a field for a domain class instance.
Examples
def b = Book.get(1)
someMethodThatMightModifyTheInstance(b)if (b.isDirty('name')) {
    def currentName = b.name
    def originalName = b.getPersistentValue('name')
    if (currentName != originalName) {
        …
    }
}Description
This method is useful mostly for audit logging or other work done in a 
beforeUpdate event callback. Hibernate caches the original state of all loaded instances for dirty checking during a flush and this method exposes that data so you can compare it with the current state.