@ManyToMany trong JPA

@ManyToMany
@JoinTable(name = "student_course", ...)
private List<Course> courses = new ArrayList<>();

student.getCourses().remove(targetCourse);

-- 1. XÓA SẠCH TOÀN BỘ 50 MÔN HỌC CỦA SINH VIÊN ĐÓ!
DELETE FROM student_course WHERE student_id = 1;
 
-- 2. INSERT LẠI 49 MÔN HỌC CÒN LẠI BẰNG 49 CÂU LỆNH INSERT RIÊNG LẺ!
INSERT INTO student_course (student_id, course_id) VALUES (1, 1);
INSERT INTO student_course (student_id, course_id) VALUES (1, 2);
...
INSERT INTO student_course (student_id, course_id) VALUES (1, 49);

package com.company.academic.domain;
 
import jakarta.persistence.Column;
import jakarta.persistence.Embeddable;
import java.io.Serializable;
import java.util.Objects;
 
@Embeddable
public class EnrollmentId implements Serializable {
 
    @Column(name = "student_id")
    private Long studentId;
 
    @Column(name = "course_id")
    private Long courseId;
 
    public EnrollmentId() {}
 
    public EnrollmentId(Long studentId, Long courseId) {
        this.studentId = studentId;
        this.courseId = courseId;
    }
 
    public Long getStudentId() { return studentId; }
    public Long getCourseId() { return courseId; }
 
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof EnrollmentId that)) return false;
        return Objects.equals(studentId, that.studentId) &&
               Objects.equals(courseId, that.courseId);
    }
 
    @Override
    public int hashCode() {
        return Objects.hash(studentId, courseId);
    }
}

package com.company.academic.domain;
 
import jakarta.persistence.*;
import java.time.Instant;
import java.util.Objects;
 
@Entity
@Table(name = "student_course_enrollments")
public class Enrollment {
 
    @EmbeddedId
    private EnrollmentId id;
 
    @ManyToOne(fetch = FetchType.LAZY)
    @MapsId("studentId") // Map trực tiếp với field studentId trong EnrollmentId
    @JoinColumn(name = "student_id")
    private Student student;
 
    @ManyToOne(fetch = FetchType.LAZY)
    @MapsId("courseId") // Map trực tiếp với field courseId trong EnrollmentId
    @JoinColumn(name = "course_id")
    private Course course;
 
    // Các thuộc tính nghiệp vụ mở rộng (Metadata)
    @Column(name = "enrolled_at", nullable = false)
    private Instant enrolledAt = Instant.now();
 
    @Column(name = "grade")
    private String grade;
 
    @Enumerated(EnumType.STRING)
    @Column(name = "status", nullable = false)
    private EnrollmentStatus status = EnrollmentStatus.ACTIVE;
 
    protected Enrollment() {}
 
    public Enrollment(Student student, Course course) {
        this.student = student;
        this.course = course;
        this.id = new EnrollmentId(student.getId(), course.getId());
    }
 
    // Getters & Setters
    public EnrollmentId getId() { return id; }
    public Student getStudent() { return student; }
    public Course getCourse() { return course; }
    public Instant getEnrolledAt() { return enrolledAt; }
    public String getGrade() { return grade; }
    public void setGrade(String grade) { this.grade = grade; }
 
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Enrollment that)) return false;
        return id != null && id.equals(that.id);
    }
 
    @Override
    public int hashCode() {
        return Objects.hashCode(id);
    }
}

@Entity
@Table(name = "students")
public class Student {
 
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
 
    @Column(nullable = false, unique = true)
    private String email;
 
    @OneToMany(mappedBy = "student", cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<Enrollment> enrollments = new HashSet<>();
 
    // Helper methods đảm bảo toàn vẹn dữ liệu
    public void enrollInCourse(Course course) {
        Enrollment enrollment = new Enrollment(this, course);
        enrollments.add(enrollment);
        course.getEnrollments().add(enrollment);
    }
 
    public void dropCourse(Course course) {
        enrollments.removeIf(e -> {
            boolean match = e.getCourse().equals(course);
            if (match) {
                course.getEnrollments().remove(e);
            }
            return match;
        });
    }
 
    public Long getId() { return id; }
    public Set<Enrollment> getEnrollments() { return Collections.unmodifiableSet(enrollments); }
}

-- DDL tạo bảng trung gian chuẩn production
CREATE TABLE student_course_enrollments (
    student_id BIGINT NOT NULL,
    course_id BIGINT NOT NULL,
    enrolled_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP NOT NULL,
    grade VARCHAR(10),
    status VARCHAR(20) DEFAULT 'ACTIVE' NOT NULL,
 
    -- Composite Primary Key: Tự động tạo B-Tree Index trên (student_id, course_id)
    PRIMARY KEY (student_id, course_id),
 
    CONSTRAINT fk_enrollment_student FOREIGN KEY (student_id) REFERENCES students(id) ON DELETE CASCADE,
    CONSTRAINT fk_enrollment_course FOREIGN KEY (course_id) REFERENCES courses(id) ON DELETE CASCADE
);
 
-- BẮT BUỘC: Tạo Index trên chiều ngược lại (course_id)!
-- Nếu không có index này, query: "Tìm tất cả sinh viên học môn Toán"
-- sẽ gây FULL TABLE SCAN trên bảng hàng triệu bản ghi!
CREATE INDEX idx_enrollment_course_id ON student_course_enrollments(course_id);