Question

我是Hibernate的新手，所以我的问题可能有些愚蠢，因为我被困住了，很乐意得到帮助。

我有两个实体：Book和Tag，结构如下：

@Entity
public class BookEntity{

@Column(nullable = false)
private String title;
@Column(nullable = false)
private String author;
private String publisher;
private int edition;
private int yearOfPublishing;

@Id
@Column(name = "isbn")
private String isbn;

@ElementCollection(fetch = FetchType.EAGER)
@OneToMany(cascade = CascadeType.ALL,fetch = FetchType.EAGER)
@JoinTable(joinColumns = { @JoinColumn(name = "isbn") },
           inverseJoinColumns = { @JoinColumn(name = "tagId") })
private List<Tag> tags;
//getters & setters

@Entity
public class Tag implements Serializable {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int tagId;
private String tagValue;
//getters & setters

插入正常，这是HQL查询：

insert into PUBLIC.BookEntity 
(author, edition, publisher, title, yearOfPublishing, isbn) 
values (?, ?, ?, ?, ?, ?)

insert into PUBLIC.Tag
(tagId, tagValue) 
values (null, ?)

选择查询看起来也很好：

select
        bookentity0_.isbn as isbn35_1_,
        bookentity0_.author as author35_1_,
        bookentity0_.edition as edition35_1_,
        bookentity0_.publisher as publisher35_1_,
        bookentity0_.title as title35_1_,
        bookentity0_.yearOfPublishing as yearOfPu6_35_1_,
        tags1_.isbn as isbn35_3_,
        tag2_.tagId as tagId3_,
        tag2_.tagId as tagId36_0_,
        tag2_.tagValue as tagValue36_0_ 
    from
        PUBLIC.BookEntity bookentity0_ 
    left outer join
        PUBLIC.BookEntity_Tag tags1_ 
            on bookentity0_.isbn=tags1_.isbn 
    left outer join
        PUBLIC.Tag tag2_ 
            on tags1_.tagId=tag2_.tagId 
    where
        bookentity0_.isbn=?

但是当我从数据库加载BookEntity时，我正在获得带有空标签列表的正确对象。数据库中的加载对象：

public T read(PK id) {
    LOG.debug("Reading by id={}", id.toString());
    return (T)getSession().get(type, id);
}

其中T是BookEntity，类型是Class，PK是String。

我做错了什么？提前谢谢。

Answer 1

首先选择isbn作为主键不是最有启发性的想法。如果用户打错并输入错误的isbn会怎么样？

其次，在我看来，你试图从书籍到标签映射多对多的关系。或者可能是一对多？

对于许多人使用：

@ManyToMany
@JoinTable(name = "book_tag",
   joinColumns = {@JoinColumn(name = "isbn")},
   inverseJoinColumns = {@JoinColumn(name = "tag_id")}
)

一对多使用：

@OneToMany(cascade = CascadeType.ALL,fetch = FetchType.EAGER)
@JoinColumn(name = "isbn", nullable = false)

但你最好用book_id替换isbn。

Hibernate OneToMany。从DB加载时获取空列表

1 个答案: