如何在jpa中使用表之间的关系

Question

我正在通过使用在线教程和尝试可能的示例来自学 jpa，但现在我对如何使用表之间的关系有点困惑。我有 3 个具有 @Entity 注释的类，这意味着 jpa 将基于这些类创建表。我在 Student、Course、Booking 类中有 id 字段，它们将是各个表的主键。 我需要的帮助是，在 Booking 类中有 sid 和 cid 字段，我希望它们被引用，例如 sid(Student.java)=sid(Booking.java) & cid(Course.java)=cid(Booking.java ) 并且场景是每个学生可以一个或多个预订一门或多门课程。有人可以告诉我在我的代码中如何以及在哪里使用@OnetoOne、@OnetoMany、@ManytoMany、@ManytoOne。

Student.java

package com.testapp;

import java.util.List;
import javax.persistence.*;

@Entity
public class Student{

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int sid;
private String name;
private int salary;

//Getters and Setters....
  ..

public Student() {
    super();
}

public Student(int sid, String name, float salary) {
    super();
    this.sid = sid;
    this.name = name;
    this.salary = salary;
}

public Student(String name, float salary) {
    super();
    this.name = name;
    this.salary = salary;
}
}

Course.java

package com.testapp;

import java.util.List;
import javax.persistence.*;

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int cid;
    private String cname;
    private int price;

    //Getters and Setters....
      ..

    public Course() {
        super();
    }

    public Course(int cid, String cname, int price) {
        super();
        this.cid = cid;
        this.cname = cname;
        this.price = price;
    }
    public Course(String cname, int price) {
        super();
        this.cname = cname;
        this.price = price;
    }
}

Booking.java

package com.testapp;

import java.util.Set;
import javax.persistence.*;

@Entity
public class Booking {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int bid;
    private String date;
    private int sid;
    private int cid;

    //Getters and Setters....
      ..

    public Booking() {
        super();
    }

    public Booking(int bid, String date, int sid, int cid) {
        super();
        this.bid = bid;
        this.date= date;
        this.sid = sid;
        this.cid = cid;
    }

    public Booking(String date, int sid, int cid) {
        super();
        this.date = date;
        this.sid = sid;
        this.cid = cid;
    }
}

谢谢你..

Answer 1

只需在您的班级中定义对象，例如涉及许多课程的学生，然后您可以在学生班级上定义属性，如下所示

public class Student{
private List<Cource> cources;
}

然后 orm 检测到关系，而且你在 JPA 中有 @OneToMant @ManyToMany 之类的注释

Answer 2

在您的案例中定义这种关系的最佳方式是 Student 和 Course 将与 Booking 具有 OneToMany 关系。并且 Booking 将与 Student 和 Course 有 ManyToOne 关系

学生.java

@OneToMany(mappedBy = "student", cascade = CascadeType.ALL, orphanRemoval = true)
public Set< Booking >   getBookings() {
    return bookings;
}

课程.java

@OneToMany(mappedBy = "course", cascade = CascadeType.ALL, orphanRemoval = true)
public Set<Booking>   getBookings() {
    return bookings;
}

Booking.java

    @Entity
    public class Booking {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int bid;
    private String date;
    private Student student;
    private Course course;

    @Id
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "sid")
    public Student getStudent() {
       return student;
    }

    @Id
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "cid")
    public Course getCourse() {
       return course;
    }
    //Getters and Setters....
      ..

    public Booking() {
        super();
    }
}

Answer 3

您不应该在 JPA 中使用其他实体的主键！

使用 @ManyToOne 和 Student 以及 Cource 代替 sid 和 cid。