JPA, как внести запись в Foreign key

//Первичный ключ
@Id
@Column (name = "ID_READER")
private int id_reader;
 
@Column (name = "READER")
private String reader;
 
@OneToMany(mappedBy="reader", cascade=CascadeType.ALL)
    private List<Book> book;
     public List<Book> getBook() {
        return book;
    }
public void setBook(List<Book> book) {
        this.book=book;
    }

//Первичный ключ
@Id
@Column(name ="ID_BOOK")
private int id_book;
 
@Column (name = "TITLE")
private String title;
 
@Column (name = "AUTHOR")
private String author;
 
@ManyToOne
@JoinColumn(name = "id_reader")
private Reader reader;
 
public Reader getReader () {
        return reader;
    }
    
public void setReader(Reader id_reader) {
this.reader = id_reader;
    }

em.getTransaction().begin();
            b = new Book();
            r = new Reader();
          //Добавляем книгу
            b.setTitle(title);
            b.setAuthor(author);
          //Указываем какому читателю будет принадлежать.
            r.setIdReader(id);
            b.setReader(r);
         //Делаем запись во вторую таблицу
            em.persist(b);
            em.flush();
em.getTransaction().commit();

java.lang.IllegalStateException: During synchronization a new object was found through a relationship that was not marked cascade PERSIST: labafour.Reader@10a46e2.