Discards any changes that have been made to a persistent instance.
def b = Book.get(1)
b.title = "Blah"
b.discard() // changes won't be applied now
method informs the persistence context that the instance should not be saved. The
method is equivalent to using Hibernate's evict
Note that this method will not clean or reset the object with the original values; it will just prevent it from being automatically saved by Grails.