单向多toone:为什么瞬态实例用cascadtype保存.除了cascatype . persist以外的所有类型

Question

当我尝试运行以下代码时，我得到org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: safu.Publisher异常:

public class SafuClient {
    public static void main(String[] args) {
        Session session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();
            Publisher publisher = new Publisher("ABC", "ABC Co.");
            Book book = new Book("1-932394-88-5", "Safu", publisher);
            session.save(book);        
            session.getTransaction().commit();
    }
}

---

我有一个从Book到Publisher的多对一关系。Book和Publisher实体如下:

---

@Entity
public class Book {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)   
    private Long id;
    private String isbn;
    private String name;
    @ManyToOne(cascade={CascadeType.PERSIST})
    @JoinColumn(name="publisher_id")
    private Publisher publisher;    
    public Book() {}
    public Book(String isbn, String name, Publisher publisher) {
        this.isbn = isbn;
        this.name = name;
        this.publisher = publisher;
    }
}

---

@Entity
public class Publisher {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;
    private String code;
    private String name;
    public Publisher() {}
    public Publisher(String code, String name) {
        this.code = code;
        this.name = name;
    }
}

---

如果我用Book实体中的@ManyToOne(cascade={CascadeType.ALL})替换@ManyToOne(cascade={CascadeType.PERSIST})，一切都工作得很好。

有人能解释一下为什么会这样吗?

Answer 1

尝试session.persist(book);来解决问题，而不是session.save(book);

Answer 2

session.save(book)方法的工作原理是从Book实体中的@ManyToOne注释中删除级联属性(cascade={CascadeType.PERSIST})，并在publisher字段中添加@Cascade (value=CascadeType.SAVE_UPDATE)。

@Cascade (value=CascadeType.SAVE_UPDATE)使用Hibernate的本地注解。

---

遵循更新后的Book.java:

---

@Entity
public class Book {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)   
    private Long id;
    private String isbn;
    private String name;
    @ManyToOne
    @JoinColumn(name="publisher_id")
    @Cascade(value=CascadeType.SAVE_UPDATE)
    private Publisher publisher;    
    public Book() {}
    public Book(String isbn, String name) {
        this.isbn = isbn;
        this.name = name;
    }   
    public Book(String isbn, String name, Publisher publisher) {
        this.isbn = isbn;
        this.name = name;
        this.publisher = publisher;
    }   
}