Data-Binding of Domain with Association inside Command Object fails (HibernateException)

class Author { String name }
class Book { Author author }

class BookCommand { Book book }

def update = { BookCommand bc -> [scaffolded update code] }

<g:select name="book.author.id" from="${dbt.Author.list()}" optionKey="id" value="${bookInstance?.author?.id}"  />

def a1 = new Author(name: "a1").save()
def a2 = new Author(name: "a2").save()
def a3 = new Author(name: "a3").save()
new Book(author: a1).save()
new Book(author: a2).save()
new Book(author: a3).save()

2010-06-10 14:49:12,623 [http-8080-1] ERROR errors.GrailsExceptionResolver  - identifier of an instance of dbt.Author was altered from 1 to 2
org.hibernate.HibernateException: identifier of an instance of dbt.Author was altered from 1 to 2
	at $Proxy10.flush(Unknown Source)
	at dbt.BookController$_closure7.doCall(BookController.groovy:80)
	at dbt.BookController$_closure7.call(BookController.groovy)
	at java.lang.Thread.run(Thread.java:619)

def bc = new BookCommand(); bindData(bc, params)

println Author.list().dump()