JPA Hibernate中多实体关联的建模：使用中间实体与复合主键

import java.io.Serializable;
import java.util.Objects;
import javax.persistence.Column;
import javax.persistence.Embeddable;

@Embeddable
public class CourseRatingKey implements Serializable {
    @Column(name = "student_id")
    private Long studentId;

    @Column(name = "course_id")
    private Long courseId;

    public CourseRatingKey() {}

    public CourseRatingKey(Long studentId, Long courseId) {
        this.studentId = studentId;
        this.courseId = courseId;
    }

    // Getters and Setters
    public Long getStudentId() { return studentId; }
    public void setStudentId(Long studentId) { this.studentId = studentId; }
    public Long getCourseId() { return courseId; }
    public void setCourseId(Long courseId) { this.courseId = courseId; }

    // 必须重写 equals 和 hashCode 方法，这是复合主键的关键
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        CourseRatingKey that = (CourseRatingKey) o;
        return Objects.equals(studentId, that.studentId) &&
               Objects.equals(courseId, that.courseId);
    }

    @Override
    public int hashCode() {
        return Objects.hash(studentId, courseId);
    }
}

import javax.persistence.*;

@Entity
@Table(name = "course_rating") // 映射到数据库中的联结表
public class CourseRating {

    @EmbeddedId
    private CourseRatingKey id; // 使用复合主键类

    // 其他属性，例如评分
    @Column(name = "rating")
    private int rating;

    // ... 构造函数、Getter/Setter 方法
}

import javax.persistence.*;
import java.util.Objects; // 导入Objects类

@Entity
@Table(name = "course_rating")
public class CourseRating {

    @EmbeddedId
    private CourseRatingKey id;

    @ManyToOne
    @MapsId("studentId") // 将此ManyToOne关系的外键映射到CourseRatingKey中的"studentId"部分
    @JoinColumn(name = "student_id") // 实际的数据库列名
    private Student student;

    @ManyToOne
    @MapsId("courseId") // 将此ManyToOne关系的外键映射到CourseRatingKey中的"courseId"部分
    @JoinColumn(name = "course_id") // 实际的数据库列名
    private Course course;

    @Column(name = "rating")
    private int rating;

    public CourseRating() {}

    public CourseRating(Student student, Course course, int rating) {
        this.student = student;
        this.course = course;
        this.rating = rating;
        // 在构造函数中初始化复合主键，确保其与关联实体的主键一致
        this.id = new CourseRatingKey(student.getId(), course.getId());
    }

    // Getters and Setters
    public CourseRatingKey getId() { return id; }
    public void setId(CourseRatingKey id) { this.id = id; }
    public Student getStudent() { return student; }
    public void setStudent(Student student) { this.student = student; }
    public Course getCourse() { return course; }
    public void setCourse(Course course) { this.course = course; }
    public int getRating() { return rating; }
    public void setRating(int rating) { this.rating = rating; }

    // 同样，equals和hashCode对于实体也很重要
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        CourseRating that = (CourseRating) o;
        return Objects.equals(id, that.id);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
}

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

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

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    // 与CourseRating建立一对多关系
    @OneToMany(mappedBy = "student", cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<CourseRating> ratings = new HashSet<>();

    public Student() {}
    public Student(String name) { this.name = name; }

    // Getters and Setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public Set<CourseRating> getRatings() { return ratings; }
    public void setRatings(Set<CourseRating> ratings) { this.ratings = ratings; }

    // 辅助方法，用于方便地添加和移除评分
    public void addRating(CourseRating rating) {
        ratings.add(rating);
        rating.setStudent(this);
        // 确保复合主