我有以下实体

@Entity
public class Note {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "NOTE_ID")
private long id;

//bi-directional many-to-one association to book
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "BOOK_ID", referencedColumnName = "BOOK_ID" )
private Book book;

}

@Entity
public class Book{

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "BOOK_ID")
private long id;

//bi-directional one-to-many association to note
@OneToMany( mappedBy="book", cascade = { CascadeType.ALL }, fetch = FetchType.LAZY )
private List<Note> notes;

}

我想知道的是
有人说，从双方更新双向关系是不好的。原因是什么？
如果我想更新双方的关系，我该怎么做？使用这种格式，当我从书本实体添加一个“注解”时，注解实体中的数据被保存，但没有书本id。如何排列设置器以便能够从两端保存？
先谢谢你