可以向@ManyToMany Hibernate 额外表添加额外字段吗?

问题描述

我有这两个类(表)

@Entity
@Table(name = "course")
public class Course {

    @Id
    @Column(name = "courseid")
    private String courseId;
    @Column(name = "coursename")
    private String courseName;
    @Column(name = "vahed")
    private int vahed;
    @Column(name = "coursedep")
    private int dep;
    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "student_course", joinColumns = @JoinColumn(name = "course_id"),  inverseJoinColumns = @JoinColumn(name = "student_id"))
    private Set<Student> student = new HashSet<Student>();
//Some setter and getter

还有这个:

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

        @Id
        @Column(name="studid")
        private String stId;
        @Column(nullable = false, name="studname")
        private String studName;
        @Column(name="stmajor")
        private String stMajor;
        @Column(name="stlevel", length=3)
        private String stLevel;
        @Column(name="stdep")
        private int stdep;

        @ManyToMany(fetch = FetchType.LAZY)
        @JoinTable(name = "student_course"
                ,joinColumns = @JoinColumn(name = "student_id")
                ,inverseJoinColumns = @JoinColumn(name = "course_id")
        )
        private Set<Course> course = new HashSet<Course>();
//Some setter and getter

运行此代码后，在数据库(student_course)中创建了一个额外的表，现在我想知道如何在该表中添加额外的字段，例如(Grade、Date 和 ...(我的意思是 student_course 表))我看到了一些解决方案，但我不喜欢它们，而且我对它们有一些问题:

After running this code an extra table was created in database(student_course), now I wanna know how can I add extra field in this table like (Grade, Date , and ... (I mean student_course table))I see some solution but I don't like them, Also I have some problem with them:

第一个样本

推荐答案

如果在链接表 (STUDENT_COURSE) 上添加额外的字段，则必须根据 skaffman 的回答或其他方法选择一种方法，如下所示.

If you add extra fields on a linked table (STUDENT_COURSE), you have to choose an approach according to skaffman's answer or another as shown bellow.

有一种方法，其中链接表 (STUDENT_COURSE) 的行为类似于 @Embeddable，根据:

There is an approach where the linked table (STUDENT_COURSE) behaves like a @Embeddable according to:

@Embeddable
public class JoinedStudentCourse {

    // Lets suppose you have added this field
    @Column(updatable=false)
    private Date joinedDate;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="STUDENT_ID", insertable=false, updatable=false)
    private Student student;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="COURSE_ID", insertable=false, updatable=false)
    private Course course;

    // getter's and setter's

    public boolean equals(Object instance) {
        if(instance == null)
            return false;

        if(!(instance instanceof JoinedStudentCourse))
            return false;

        JoinedStudentCourse other = (JoinedStudentCourse) instance;
        if(!(student.getId().equals(other.getStudent().getId()))
            return false;

        if(!(course.getId().equals(other.getCourse().getId()))
            return false;

        // ATT: use immutable fields like joinedDate in equals() implementation
        if(!(joinedDate.equals(other.getJoinedDate()))
            return false;

        return true;
    }

    public int hashcode() {
        // hashcode implementation
    }

}

因此您将同时拥有学生和课程课程

So you will have in both Student and Course classes

public class Student {

    @CollectionOfElements
    @JoinTable(
        table=@Table(name="STUDENT_COURSE"),
        joinColumns=@JoinColumn(name="STUDENT_ID")
    )
    private Set<JoinedStudentCourse> joined = new HashSet<JoinedStudentCourse>();

}

public class Course {

    @CollectionOfElements
    @JoinTable(
        table=@Table(name="STUDENT_COURSE"),
        joinColumns=@JoinColumn(name="COURSE_ID")
    )
    private Set<JoinedStudentCourse> joined = new HashSet<JoinedStudentCourse>();

}

记住:@Embeddable 类的生命周期绑定到拥有的实体类(Student 和 Course)，所以要注意它.

remember: @Embeddable class has its lifecycle bound to the owning entity class (Both Student and Course), so take care of it.

建议:由于@ManyToMany 映射的一些限制 - 级联操作，Hibernate 团队支持这两种方法(@OneToMany(skaffman 的回答)或@CollectionsOfElements).

advice: Hibernate team suppports these two approachs (@OneToMany (skaffman's answer) or @CollectionsOfElements) due some limitations in @ManyToMany mapping - cascade operation.

问候，

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