java - EntityManager 抛出 @OnetoMany 映射异常

Question

我目前正在尝试以双向关系映射这两个实体，但出现如图所示的异常。实体

Book.java:

@Entity
@Table(name = "Book")
public class Book {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long id;


    @OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL, mappedBy="bookId")
    private Collection<Author> authors;

    public Collection<Author> getAuthors() {
        return authors;
    }

        ...
}

作者.java:

@Entity
public class Author {

    @Id
    @GeneratedValue
    @Column(name="id")
    private Long id;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="author_id")
    private Long bookId;

        ...
}

异常(exception):

Exception in thread "main" javax.persistence.PersistenceException: [PersistenceUnit: BookStore] Unable to build EntityManagerFactory
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:914)
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:889)
    at org.hibernate.ejb.HibernatePersistence.createEntityManagerFactory(HibernatePersistence.java:56)
    at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:63)
    at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:47)
    at org.hibstore.dao.DAOTest.main(DAOTest.java:10)
Caused by: org.hibernate.AnnotationException: @OneToOne or @ManyToOne on org.hibstore.domain.Author.bookId references an unknown entity: java.lang.Long
    at org.hibernate.cfg.ToOneFkSecondPass.doSecondPass(ToOneFkSecondPass.java:109)
    at org.hibernate.cfg.Configuration.processEndOfQueue(Configuration.java:1514)

有什么想法吗？

Answer 1

引用this post了解如何使用JoinColumn。

详细说明:您的 Author 实体必须指向 Book 实体，而不是其 id。这就是使用 JPA 的优点:您使用实体，而不是它们的键。

这是一个例子:

@Entity
@Table(name = "Book")
public class Book {
  @OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL, mappedBy="book")
  private Collection<Author> authors;
  // ...
}

@Entity
public class Author {
  @ManyToOne(fetch=FetchType.LAZY)
  @JoinColumn(name="book_id")
  private Book book;
  // ...
}

为什么每个作者有一本书(连接列指向此处的书)是值得怀疑的，您可能应该重新考虑您的数据模型:多对多关系可能更有意义，但您将需要第三种关系表(有如何相应设置 JPA 实体的示例)。