Question

我几天前做过这个，我有一个这样的中间表：

-------------------------
|COURSE_ID | STUDENT_ID |
------------------------
|0         |1           |
|0         |2           |
|1         |1           |
|1         |2           |
-------------------------

但后来我想给它添加字段（并没有将它提交给github¬，我很着急）并最终打破了整个事情...... 长话短说我再次开始这个例子。我的代码现在创建了两个表“courses”和“students”，但它们没有反映出它们之间的关系，我的意思是没有创建中间表;虽然我可以向对象世界中的每个学生请求课程，反之亦然，但在数据库世界中，这一点都没有反映出来。这是我的代码：

学生班 包模型;

import java.util.HashSet;
import javax.persistence.JoinColumn;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;
import java.util.Set;

@Entity
@Table(name="STUDENTS")
public class Student
{
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int id ;

    private String name ;

    @ManyToMany(targetEntity=Course.class, mappedBy="courseName", fetch=FetchType.EAGER)
    private Set<Course> courses ;

    public Student(String myName)
    {
        this.name = myName ;
        courses = new HashSet<Course>() ;
    }

    public void addCourse(Course myCourse)
    {
        this.courses.add(myCourse) ;
    }


    @Column(name="STUDENT_ID")
    public int getStudent_id()
    {
        return this.id ;
    }


    @JoinTable(name="COURSE_STUDENTS",
        joinColumns=
            @JoinColumn(name="STUDENT_ID"),
        inverseJoinColumns=
            @JoinColumn(name="COURSE_ID")
                )
    public Set<Course> getCourses()
    {
        return this.courses ;
    }

    @Column(name="STUDENT_NAME")
    public String getStudent_name()
    {
        return this.name ;
    }

}

课程类

package model;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="COURSES")
public class Course
{
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int id ;

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

    @ManyToMany(targetEntity=Student.class, mappedBy="name", fetch=FetchType.EAGER)
    private Set<Student> students ;


    public Course(String myName)
    {
        this.courseName = myName ;
        this.students = new HashSet<Student> () ;
    }

    public void addStudent(Student myStudent)
    {
        this.students.add(myStudent) ;
    }

    @ManyToMany(mappedBy="courses")
    public Set<Student> getCourse_students()
    {
        return this.students ;
    }

    @Column(name="COURSE_ID")
    public int getCourse_id()
    {
        return id ;
    }
}

测试此内容的主要类

package controller;

import javax.persistence.EntityManager;

import db.EntityManagerHelper;
import model.Student;
import model.Course;

public class Main {

    public static void main(String[] args) {

        EntityManager entityManager= EntityManagerHelper.getEntityManager();

        Course course_kK = new Course("K1000") ;
        Course course_mK = new Course("M1000") ;
        Course course_rK = new Course("R1000") ;

        Student student_rick = new Student("Rick") ;
        student_rick.addCourse(course_rK);
        student_rick.addCourse(course_mK);

        Student student_rock = new Student("Rock") ;
        student_rock.addCourse(course_rK);
        student_rock.addCourse(course_kK);

        Student student_jack = new Student("Jack") ;
        student_jack.addCourse(course_mK);
        student_jack.addCourse(course_kK);

        course_kK.addStudent(student_rock);
        course_kK.addStudent(student_jack);
        course_mK.addStudent(student_jack);
        course_mK.addStudent(student_rick);
        course_rK.addStudent(student_rock);
        course_rK.addStudent(student_rick);

        System.out.println("course_rK:id = "+course_rK.getCourse_id());
        System.out.println("course_kK:id = "+course_kK.getCourse_id());
        System.out.println("course_mK:id = "+course_mK.getCourse_id());

        EntityManagerHelper.beginTransaction();

        entityManager.persist(course_rK);
        entityManager.persist(course_kK);
        entityManager.persist(course_mK);

        entityManager.persist(student_rick);
        entityManager.persist(student_rock);
        entityManager.persist(student_jack);

        entityManager.flush();
        EntityManagerHelper.commit();   

        System.out.println("student_jack:id = "+student_jack.getStudent_id());
        System.out.println("student_rock:id = "+student_rock.getStudent_id());
        System.out.println("student_gast:id = "+student_rick.getStudent_id());
    }
}

如何在“制动”多对多关系的两个实体之间创建该中间表？

Answer 1

这是详细的答案。我不得不做几行修改......我再次检查了this教程，就像我在原来的问题中做的那样，我第一次看到了。

在哪里说

@Column(name="COURSE_ID")

我把它改为

@Column(name="COURSE_ID", unique=true, nullable=false)

在哪里说

@Column(name="STUDENT_ID")

我把它改为

@Column(name="STUDENT_ID", unique=true, nullable=false)

在哪里说

@ManyToMany(targetEntity=Course.class, mappedBy="courseName", fetch=FetchType.EAGER)

我把它改为

@ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)

在哪里说

@ManyToMany(mappedBy="courses")

我把它改为

@ManyToMany(fetch = FetchType.LAZY, mappedBy = "courses")

它说的地方

 @JoinTable(name="COURSE_STUDENTS",
        joinColumns=
            @JoinColumn(name="STUDENT_ID"),
        inverseJoinColumns=
            @JoinColumn(name="COURSE_ID")
                )
    public Set<Course> getCourses()
    {
        return this.courses ;
    }

我把它改为

@JoinTable(name = "STUDENT_COURSE", joinColumns = {
        @JoinColumn(name = "STUDENT_ID", nullable = false, updatable = false) },
        inverseJoinColumns = { @JoinColumn(name = "COURSE_ID",
                nullable = false, updatable = false) }
            )
public Set<Course> getCourses()
{
    return this.courses ;
}

如何使用@ManyToMany和hibernate在实体之间创建一个中间表？

1 个答案: