一对一外键映射

Question

在 Book 和 author 之间的以下映射中，我使用 book table 中的外键列 author_id 完成了一对一映射，并且关系被标记为 optional false from book->author ，但是当我查询时 session.createQuery("from Book").list();

1) 当标记为可选为 false 时，它正在急切地获取作者信息，为什么不代理它，有人可以解释为什么它不能像在主键连接列中那样使用外键方法吗？

Hibernate: select book0_.BOOK_ID as BOOK_ID1_1_, book0_.AUTHOR_ID as AUTHOR_I5_1_, book0_.description as descript2_1_, book0_.PUBLISHED as PUBLISHE3_1_, book0_.title as title4_1_ from BOOK book0_

Hibernate: select author0_.AUTHOR_ID as AUTHOR_I1_0_0_, author0_.email as email2_0_0_, author0_.name as name3_0_0_ from AUTHOR author0_ where author0_.AUTHOR_ID in (?, ?, ?)

Hibernate: select author0_.AUTHOR_ID as AUTHOR_I1_0_0_, author0_.email as email2_0_0_, author0_.name as name3_0_0_ from AUTHOR author0_ where author0_.AUTHOR_ID in (?, ?, ?)

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

     private long id;
        private String title;
        private String description;
        private Date publishedDate;

        private Author author;

        public Book() {
        }

        @Id
        @Column(name = "BOOK_ID")
        @GeneratedValue
        public long getId() {
            return id;
        }

        public void setId(long id) {
            this.id = id;
        }

        public String getTitle() {
            return title;
        }

        public void setTitle(String title) {
            this.title = title;
        }

        public String getDescription() {
            return description;
        }

        public void setDescription(String description) {
            this.description = description;
        }

        @Temporal(TemporalType.DATE)
        @Column(name = "PUBLISHED")
        public Date getPublishedDate() {
            return publishedDate;
        }

        public void setPublishedDate(Date publishedDate) {
            this.publishedDate = publishedDate;
        }

        @OneToOne(cascade = CascadeType.ALL,optional=false)
        @JoinColumn(name = "AUTHOR_ID")
        public Author getAuthor() {
            return author;
        }

        public void setAuthor(Author author) {
            this.author = author;
        }
    }

@Entity
@BatchSize(size=3)
@Table(name = "AUTHOR")
public class Author {
    private long id;
    private String name;
    private String email;


    public Author() {
    }

    public Author(String name, String email) {
        this.name = name;
        this.email = email;
    }

    @Id
    @Column(name = "AUTHOR_ID")
    @GeneratedValue
    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}

2) 当我从 author->book 进行双向查询时，运行查询 session.createQuery("from Author").list();

它首先按预期从 Author 获取记录，然后运行如下使用 2 个连接的单个查询。为什么它必须进行连接而不是直接使用作者 ID 查询图书 table？

Hibernate: select author0_.AUTHOR_ID as AUTHOR_I1_0_, author0_.email as email2_0_, author0_.name as name3_0_ from AUTHOR author0_

Hibernate: select book0_.BOOK_ID as BOOK_ID1_1_2_, book0_.AUTHOR_ID as AUTHOR_I5_1_2_, book0_.description as descript2_1_2_, book0_.PUBLISHED as PUBLISHE3_1_2_, book0_.title as title4_1_2_, author1_.AUTHOR_ID as AUTHOR_I1_0_0_, author1_.email as email2_0_0_, author1_.name as name3_0_0_, book2_.BOOK_ID as BOOK_ID1_1_1_, book2_.AUTHOR_ID as AUTHOR_I5_1_1_, book2_.description as descript2_1_1_, book2_.PUBLISHED as PUBLISHE3_1_1_, book2_.title as title4_1_1_ from BOOK book0_ inner join AUTHOR author1_ on book0_.AUTHOR_ID=author1_.AUTHOR_ID left outer join BOOK book2_ on author1_.AUTHOR_ID=book2_.AUTHOR_ID where book0_.AUTHOR_ID=?

Hibernate: select book0_.BOOK_ID as BOOK_ID1_1_2_, book0_.AUTHOR_ID as AUTHOR_I5_1_2_, book0_.description as descript2_1_2_, book0_.PUBLISHED as PUBLISHE3_1_2_, book0_.title as title4_1_2_, author1_.AUTHOR_ID as AUTHOR_I1_0_0_, author1_.email as email2_0_0_, author1_.name as name3_0_0_, book2_.BOOK_ID as BOOK_ID1_1_1_, book2_.AUTHOR_ID as AUTHOR_I5_1_1_, book2_.description as descript2_1_1_, book2_.PUBLISHED as PUBLISHE3_1_1_, book2_.title as title4_1_1_ from BOOK book0_ inner join AUTHOR author1_ on book0_.AUTHOR_ID=author1_.AUTHOR_ID left outer join BOOK book2_ on author1_.AUTHOR_ID=book2_.AUTHOR_ID where book0_.AUTHOR_ID=?

Hibernate: select book0_.BOOK_ID as BOOK_ID1_1_2_, book0_.AUTHOR_ID as AUTHOR_I5_1_2_, book0_.description as descript2_1_2_, book0_.PUBLISHED as PUBLISHE3_1_2_, book0_.title as title4_1_2_, author1_.AUTHOR_ID as AUTHOR_I1_0_0_, author1_.email as email2_0_0_, author1_.name as name3_0_0_, book2_.BOOK_ID as BOOK_ID1_1_1_, book2_.AUTHOR_ID as AUTHOR_I5_1_1_, book2_.description as descript2_1_1_, book2_.PUBLISHED as PUBLISHE3_1_1_, book2_.title as title4_1_1_ from BOOK book0_ inner join AUTHOR author1_ on book0_.AUTHOR_ID=author1_.AUTHOR_ID left outer join BOOK book2_ on author1_.AUTHOR_ID=book2_.AUTHOR_ID where book0_.AUTHOR_ID=?

Hibernate: select book0_.BOOK_ID as BOOK_ID1_1_2_, book0_.AUTHOR_ID as AUTHOR_I5_1_2_, book0_.description as descript2_1_2_, book0_.PUBLISHED as PUBLISHE3_1_2_, book0_.title as title4_1_2_, author1_.AUTHOR_ID as AUTHOR_I1_0_0_, author1_.email as email2_0_0_, author1_.name as name3_0_0_, book2_.BOOK_ID as BOOK_ID1_1_1_, book2_.AUTHOR_ID as AUTHOR_I5_1_1_, book2_.description as descript2_1_1_, book2_.PUBLISHED as PUBLISHE3_1_1_, book2_.title as title4_1_1_ from BOOK book0_ inner join AUTHOR author1_ on book0_.AUTHOR_ID=author1_.AUTHOR_ID left outer join BOOK book2_ on author1_.AUTHOR_ID=book2_.AUTHOR_ID where book0_.AUTHOR_ID=?

Answer 1

Hibernate 仅代理标记为 LAZY=true 的模型。以下线程可能会帮助您回答某些问题。 Hibernate lazy loading, optional = false

Answer 2

1) ToOne 关联在 JPA 中默认是急切的。使其变得懒惰：

@OneToOne(cascade = CascadeType.ALL, optional = false, , fetch = FetchType.LAZY)

用不用@PrimaryKeyJoinColumn都无所谓

2) 没错，这是不必要的，Hibernate 可以做得更好；请看这个。关键是 Hibernate 希望在急切地获取关联实体时避免额外的查询。

那么，这里发生了什么？

首先，您可能没有在 Author 方面声明关联是强制性的（这就是 left outer join 的来源）。

其次，Hibernate 通过提供的作者 ID 阅读书籍（对于第一个查询返回的每个作者）。如果您手动执行，您将得到相同的查询：

select b from Book b where b.author.id = :authorId

Hibernate使用第一个join table读这本书，第二个join读这本书的作者，第三个读作者的书。

这里不需要第三个连接 table，我希望 Hibernate 在未来的版本中对此进行改进。

第二个连接 table 如果您手动执行上述查询（在同一个查询中而不是在新查询中阅读图书的作者），那么第二个连接 table 是一个很好的优化，但在您的示例中是不必要的，因为所需的作者已经加载。我希望在未来的版本中也会对此进行改进。

PS Author 和 Book 之间的真实关联是一对多的。

一对一外键映射

One to One foreign key mapping

persistence

hibernate

jpa