java - hibernate 一对多。从数据库加载时获取空列表

Question

我是 Hibernate 新手，所以我的问题可能有点愚蠢，因为我被困住了，很高兴获得帮助。

我有两个实体:书籍和标签，其结构如下:

@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

<小时/>

@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，类型是 Class，PK 是 String。

我做错了什么？ 提前致谢。

Answer 1

首先，选择 isbn 作为主键并不是最有灵感的想法。如果用户输入错误并输入了错误的 isbn 怎么办？

其次，在我看来，您正在尝试将书籍与标签之间的多对多关系映射起来。或者也许是一对多？

对于多对多用途:

@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)

但是你最好用 book_id 替换 isbn。