Question

我有三个实体类; Student，Subject和StudentSubject。

Student在StudentSubject上有一对多关系，而Subject在StudentSubject上也有一对多关系。

学生班

@Entity
@Getter
@Setter
public class Student {
    @Id
    private String email;

    @OneToMany(mappedBy = "student")
    @JsonManagedReference
    private List<StudentSubject> subjects;
    //more elements
}

主题类

@Entity
@Getter
@Setter
public class Subject {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToOne
    @JsonBackReference
    private Teacher teacher;

    @OneToMany(mappedBy = "student")
    @JsonManagedReference
    private List<StudentSubject> students;
    //more elements
}

StudentSubject类

@Entity
@IdClass(StudentSubjectId.class)
@Getter
@Setter
public class StudentSubject implements Serializable {

    //Primary keys
    @Id
    @Column(name = "subject_id")
    Long subjectId;
    @Id
    @Column(name = "student_email")
    String studentEmail;

    String uid;

    @ManyToOne
    @JsonBackReference
    @JoinColumn(name = "subject_id", insertable = false, updatable = false)
    private Subject subject;
    @ManyToOne
    @JsonBackReference
    @JoinColumn(name = "student_email", insertable = false, updatable = false)
    private Student student;
}

我有3个班级，而不是2个班级，因为每个学生科目对都有特定的属性。因此这种安排。

当我从存储库中读取主题时，

Subject subject = subjectRepository.findByNameAndTeacher(subjectName, teacher);
subject.getStudents();

除了学生名单外，所有细节都是正确的。它总是空的。（通过添加断点来检查）

Hibernate / JPA执行的查询是

获得主题（？）

select
    subject0_.id as id1_3_,
    subject0_.name as name2_3_,
    subject0_.teacher_email as teacher_3_3_ 
from
    subject subject0_ 
    left outer join
        teacher teacher1_ 
        on subject0_.teacher_email = teacher1_.email 
where
    subject0_.name =? 
    and teacher1_.email =?

选择学生列表（？）

select
    students0_.student_email as student_1_2_0_,
    students0_.subject_id as subject_2_2_0_,
    students0_.student_email as student_1_2_1_,
    students0_.subject_id as subject_2_2_1_,
    students0_.uid as uid3_2_1_,
    subject1_.id as id1_3_2_,
    subject1_.name as name2_3_2_,
    subject1_.teacher_email as teacher_3_3_2_,
    teacher2_.email as email1_5_3_,
    teacher2_.name as name2_5_3_ 
from
    student_subject students0_ 
    left outer join
        subject subject1_ 
        on students0_.subject_id = subject1_.id 
    left outer join
        teacher teacher2_ 
        on subject1_.teacher_email = teacher2_.email 
where
    students0_.student_email =?

等等。

我认为这里的问题是最后一个where子句被错误地添加，并且表中的公共属性不会显示一次。我该如何解决这个问题？

Answer 1

您的地图有拼写错误。在Subject类中，它应该是@OneToMany(mappedBy = "subject")而不是mappedBy="student"因此你的错误的where子句。

这就是它使用的原因 where students0_.student_email =? 代替 where students0_.subject_id =?因为它认为从主题获取学生的方式是通过student_email列，如您的映射所示。

Answer 2

您尚未指定提取类型。这应该解决它。

@OneToMany(mappedBy = "student", fetch = FetchType.EAGER)
@JsonManagedReference
private List<StudentSubject> students;

Spring启动：JPA错误地添加了where子句

2 个答案: