Spring Data Jpa

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <!-- for mongodb -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-mongodb</artifactId>
        </dependency>

        <!-- for postgresql -->
        <dependency>
            <groupId>org.postgresql</groupId>
            <artifactId>postgresql</artifactId>
            <version>${postgresql.version}</version>
        </dependency>

        <!-- for mysql -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.32</version>
        </dependency>

@Entity
@Table(name="course")
public class Course {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;

  private String name;

  @Column
  private Integer credit;

  @OneToOne
  @JoinColumn(name = "teacher_id")
  private Teacher teacher;

  @OneToMany(mappedBy = "course")
  private List<Enrollment> enrollmentList;

  // setter and getter ...
}

@Entity
@Table(name = "enrollment")
public class Enrollment {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;

  @ManyToOne
  private Course course;

  @ManyToOne
  private Student student;

  private Integer score;

  // setter and getter ...
}

@Entity
@Table(name = "student")
public class Student {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;

  private String name;

  private LocalDate dob;

  @OneToMany(mappedBy = "student")
  List<Enrollment> enrollmentList;

  // setter and getter ...
}

@Document("enrollment_history")
public class History {
  @Id
  private String id;

  private String action;
  private String studentName;
  private String courseName;

  // setter and getter ...
}

// package ...

// import ...Enrollment;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface EnrollmentRepository extends JpaRepository<Enrollment, Long> {
}

// package ...

// import ...Student;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

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

// package ...

// import ...Teacher;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface TeacherRepository extends JpaRepository<Teacher, Long> {
  // select * from teachers where age < 40 and id > 1000;
  List<Teacher> findByAgeLessThan(int age);
  List<Teacher> findByAge(int age);

  // @Query(value = "select t from Teacher t where t.age < ? and t.id > ?", nativeQuery = true) // native sql query
  @Query(value = "select t from Teacher t where t.age < :age and t.id > :id") // jpql
  List<Teacher> a(int age, long id);
}

public interface HistoryRepository extends MongoRepository<History, String> {
  List<History> findByAction(String action);
  List<History> findByCourseName(String courseName);
}

@Service
public class EnrollmentServiceImpl implements EnrollmentService {
  @Autowired
  EnrollmentRepository enrollmentRepository;

  @Autowired
  HistoryRepository historyRepository;

  @Override
  @Transactional
  public Enrollment enroll(Student student, Course course) {
    History history = new History();
    // history.set....
    historyRepository.save(history);

    Enrollment enrollment = new Enrollment();
    // enrollment.set...
    return enrollmentRepository.save(enrollment);
  }

  @Override
  public EnrollmentVO getEnrollment(Long id) {
    Enrollment enrollment = enrollmentRepository.findById(id).orElse(null);
    EnrollmentVO vo = new EnrollmentVO();
    // vo.set...
    return vo;
  }

  @Override
  public List<Enrollment> getEnrollmentByCourseId(Long courseId) {
    Enrollment enrollmentExample = new Enrollment();
    Course c = new Course();
    c.setId(courseId);
    enrollmentExample.setCourse(c);
    Example<Enrollment> example = Example.of(enrollmentExample);
    List<Enrollment> enrollments = enrollmentRepository.findAll(example);
    List<EnrollmentVO> voList = new ArrayList<>();
    enrollments.forEach(enrollment -> {
      EnrollmentVO vo = new EnrollmentVO();
      // vo.setId(enrollment.getId());
      // vo.setCourseName(enrollment.getCourse().getName());
      // vo.setStudentName(enrollment.getStudent().getName());
      voList.add(vo);
    })
    return voList;
  }

  @Override
  public List<EnrollmentVO> getAll(int page, int rows) {
    Pageable pageRequest = PageRequest.of(page, rows);
    Page<Enrollment> p = enrollmentRepository.findAll(pageRequest);
    List<Enrollment> enrollments = p.getContent();
    List<EnrollmentVO> voList = enrollments.stream().map(enrollment -> {
      EnrollmentVO vo = new EnrollmentVO();
      // vo.set...
      return vo;
    }).toList();
    return voList;
  }
}

@Service
public class StudentServiceImpl implements StudentService {
  @Autowired
  StudentRepository studentRepository;

  @Override
  public List<StudentVO> getAllStudents() {
    return studentRepository.findAll().stream().map(entity -> {
      StudentVO temp = new StudentVO();
      BeanUtils.copyProperties(entity, temp);
      temp.setDob(Date.valueOf(entity.getDob()));
      return temp;
    }).toList();
  }
}

@Service
@Primary
public class UserServiceImpl implements UserService {
  public void login(String username) { System.out.println("in user service, use password"); }
}

@Service
public class UserServiceImpl implements UserService {
  public void login(String username) { System.out.println("in user service2, login by fingerprint"); }
}