在Spring Boot JPA中实现多对多关系数量限制的策略

// Student.java
@Entity
@NoArgsConstructor
@Getter
@Setter
public class Student extends BaseEntity { // 假设BaseEntity包含ID字段
    private String name;
    private String surname;
    @Column(name = "student_number", unique = true)
    private String number; // student number

    @JsonIgnore
    @ManyToMany(cascade = CascadeType.MERGE, fetch = FetchType.EAGER)
    @JoinTable(name = "students_courses",
               joinColumns = @JoinColumn(name = "student_id"),
               inverseJoinColumns = @JoinColumn(name = "course_id"))
    private List<Course> courseList = new ArrayList<>();
}

// Course.java
@Entity
@Getter
@Setter
public class Course extends BaseEntity {
    @Column(name = "course_name", unique = true)
    private String courseName;

    @JsonIgnore
    @ManyToMany(mappedBy = "courseList", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    private List<Student> studentList = new ArrayList<>();
}

// StudentRepository.java
@Repository
public interface StudentRepository extends JpaRepository<Student, Long> {
}

// CourseRepository.java
@Repository
public interface CourseRepository extends JpaRepository<Course, Long> {
}

// 定义自定义异常
public class CourseLimitExceededException extends RuntimeException {
    public CourseLimitExceededException(String message) {
        super(message);
    }
}

// StudentService.java
@Service
public class StudentService {

    private final StudentRepository studentRepository;
    private final CourseRepository courseRepository;

    private static final int MAX_COURSES_PER_STUDENT = 3; // 学生选课上限

    public StudentService(StudentRepository studentRepository, CourseRepository courseRepository) {
        this.studentRepository = studentRepository;
        this.courseRepository = courseRepository;
    }

    @Transactional
    public Student addCourseToStudent(Long studentId, Long courseId) {
        Student student = studentRepository.findById(studentId)
                .orElseThrow(() -> new EntityNotFoundException("Student not found with ID: " + studentId));
        Course course = courseRepository.findById(courseId)
                .orElseThrow(() -> new EntityNotFoundException("Course not found with ID: " + courseId));

        // 检查学生是否已选该课程
        if (student.getCourseList().contains(course)) {
            throw new IllegalArgumentException("Student already registered for this course.");
        }

        // 检查学生选课数量是否超出限制
        if (student.getCourseList().size() >= MAX_COURSES_PER_STUDENT) {
            throw new CourseLimitExceededException("Student cannot register for more than " + MAX_COURSES_PER_STUDENT + " courses.");
        }

        // 添加课程
        student.getCourseList().add(course);
        course.getStudentList().add(student); // 维护双向关系

        return studentRepository.save(student); // 保存学生实体，JPA会自动更新关联表
    }

    // 其他学生相关业务方法...
}

// 定义自定义异常
public class StudentLimitExceededException extends RuntimeException {
    public StudentLimitExceededException(String message) {
        super(message);
    }
}

// CourseService.java
@Service
public class CourseService {

    private final CourseRepository courseRepository;
    private final StudentRepository studentRepository;

    private static final int MAX_STUDENTS_PER_COURSE = 10; // 课程学生上限

    public CourseService(CourseRepository courseRepository, StudentRepository studentRepository) {
        this.courseRepository = courseRepository;
        this.studentRepository = studentRepository;
    }

    @Transactional
    public Course addStudentToCourse(Long courseId, Long studentId) {
        Course course = courseRepository.findById(courseId)
                .orElseThrow(() -> new EntityNotFoundException("Course not found with ID: " + courseId));
        Student student = studentRepository.findById(studentId)
                .orElseThrow(() -> new EntityNotFoundException("Student not found with ID: " + studentId));

        // 检查课程是否已包含该学生
        if (course.getStudentList().contains(student)) {
            throw new IllegalArgumentException("Course already enrolled this student.");
        }

        // 检查课程学生数量是否超出限制
        if (course.getStudentList().size() >= MAX_STUDENTS_PER_COURSE) {
            throw new StudentLimitExceededException("Course cannot enroll more than " + MAX_STUDENTS_PER_COURSE + " students.");
        }

        // 添加学生
        course.getStudentList().add(student);
        student.getCourseList().add(course); // 维护双向关系

        return courseRepository.save(course); // 保存课程实体
    }

    // 其他课程相关业务方法...
}