休眠一对多。从数据库加载时获取空列表

Question

我是 Hibernate 的新手，所以我的问题可能有点傻，因为我被困住了，很高兴得到帮助。

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

插入工作正常，这是 HQL 查询：

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

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

选择查询看起来也不错：

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，我得到了带有空标签列表的正确对象。 从数据库加载对象：

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

其中 T 是 BookEntity，type 是 Class，PK 是 String。

我做错了什么？ 提前致谢。

Answer 1

首先选择 isbn 作为主键并不是最受启发的想法。如果用户打错了isbn怎么办？

其次，在我看来，您正在尝试将多对多关系从书籍映射到标签。或者可能是一对多？

多对多用途：

@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。