带有级联的全删除孤儿的removeFrom不起作用

class Author {
    String name
    static hasMany = [books: Book]
    static constraints = {
        books cascade: \'all-delete-orphan\'
    }
    String toString() {
        return name
    }
}

class Book {
    String title
    static belongsTo = [author: Author]
    static constraints = {
    }
    String toString() {
        return title
    }
}

Bootstrap:
def a = new Author(name: \'Author0\')
a.save(flush: true, failOnError: true)      
def b = new Book(title: \'Book0\')
a.addToBooks(b).save(flush: true, failOnError: true)

class AuthorController {
    def index = {
        println(\"Controller code: \" + \"Old books by author: \" + Author.get(1).books)
        def author = Author.findByName(\"Author0\")
        def oldBooks = []
        oldBooks += author.books  // to avoid ConcurrentModificationException
        oldBooks.each {
            author.removeFromBooks(it)
        }
        author.save(flush: true, failOnError: true)
        println(\"Controller code: \" + \"New books by author: \" + Author.get(1).books)
            render(\"All done!\")
        }
    }