java - ManyToMany 注释不会在选择查询中生成联接

Question

摘要:我正在尝试使用 jpa + hibernate 构建并急切地检索单向 ManyToMany 关系。

当执行 findAll() 时，引用的实体为空，而本应有 1。

根据我的观察，hibernate 生成的 SQL(使用 show-sql 属性)缺少连接。

上下文:

• Spring 数据 - 4.0.4.RELEASE
• hibernate - 4.2.21.Final

我尝试过的一些替代方案/修复:

• 已尝试将它们切换为 OneToMany/ManyToOne 关系。
• 尝试过使用mappedBy引用Student的类来建立双向ManyToMany关系
• 已尝试更新类的 id 列，使其在连接表中不发生变化

Java 实体

@Data
@Entity
@Table(name = "student")
public class Student {

@Column(name = "student_id", unique = true, nullable = false)
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

@Data
@Entity
@Table(name = "Class")
public class Class {

@Column(name = "id", unique = true, nullable = false)
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

@ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(
        name = "class_student",
        joinColumns = @JoinColumn(name = "class_id", referencedColumnName = "id"),
        inverseJoinColumns = @JoinColumn(name = "student_id", referencedColumnName = "student_id"))
    private Set<Student> students;

DDL:

CREATE TABLE IF NOT EXISTS `student` (
    student_id INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (`student_id `)
);

CREATE TABLE IF NOT EXISTS `class` (
    `id` INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (`id`)
);

CREATE TABLE IF NOT EXISTS `class_student ` (
    student_id INT NOT NULL,
    class_id INT NOT NULL,
    PRIMARY KEY (`student_id `, `class_id`),
    CONSTRAINT `fk_student_id` FOREIGN KEY (`student_id`) REFERENCES `student` (`student_id`),
    CONSTRAINT `fk_class_id` FOREIGN KEY (`class_id`) REFERENCES `class` (`id`)
);

实际与预期:当我 classRespository.getAll() 时实际生成的 sql 似乎缺少我期望的 class + class_student 之间的连接。

它还奇怪地包括我没想到的 Student 和 class_student 之间的连接，因为它是单向定义的，而且除了可能被丢弃之外，我不知道如何处理该意外查询的结果。

实际生成的 SQL :

Hibernate: 
    select
        classin0_.id as id1_29_,
    from
        class classin0_
    INNER JOIN
       post_tag pt
    ON     p.id = pt.post_id
    INNER JOIN
        tag t
    ON     pt.tag_id = t.id
    WHERE  p.id = 1
Hibernate: 
    select
        class_student0_.class_id as id1_30_1_,
        class_student0_.student_id as featured2_10_1_,
        student1_.student_id as featured1_28_0_,
    from
        class_student class_student0_ 
    inner join
        student student1_ 
            on class_student0_.student_id=student1_.student_id 
    where
        class_student0_.id=?

我期望一个跨三个表的内部联接查询，jpa 可以将其解析回一个集合。

如有任何帮助，我们将不胜感激。

Answer 1

我已经检查了您的映射 - 它是正确的。我正在使用这些类。

@Entity
@Table(name = "student")
public class Student {

    @Column(name = "student_id", unique = true, nullable = false)
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

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

}

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

    @Column(name = "id", unique = true, nullable = false)
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "className")
    private String className;

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(
            name = "class_student",
            joinColumns = @JoinColumn(name = "class_id", referencedColumnName = "id"),
            inverseJoinColumns = @JoinColumn(name = "student_id", referencedColumnName = "student_id"))
    private Set<Student> students;

}

The actual generated sql when I classRespository.getAll() seems to be missing the join between class + class_student that I would expect. It also oddly includes a join between student and class_student that I would not expect

Hibernate 不使用 join 来连接 class 和 class_student 关系。

Hibernate 通过第一个查询获取所有类

select
    class0_.id as id1_0_,
    class0_.className as classNam2_0_ 
from
    Class class0_

然后，使用 class_student 和 student 之间的内部联接获取一个现有类(class)的所有学生。

   select
        students0_.class_id as class_id1_1_0_,
        students0_.student_id as student_2_1_0_,
        student1_.student_id as student_1_2_1_,
        student1_.name as name2_2_1_ 
    from
        class_student students0_ 
    inner join
        student student1_ 
            on students0_.student_id=student1_.student_id 
    where
        students0_.class_id=?

我无法检查您的 SQL，因为它不正确。例如，class_student表中没有这样的字段class_student0_.id=?。