studentRepository.findByStudentId(studentId)
    .orElseThrow(() -> new IllegalArgumentException("존재하지 않는 학생입니다."));

@PutMapping("/change/student/name/{studentId}")
public ResponseEntity<StudentResponseDto> changeStudentName(
    @PathVariable String studentId,
    @RequestBody StudentRequestDto requestDto
) {
    return ResponseEntity.ok(studentService.updateStudentName(studentId, requestDto));
}

import org.springframework.web.bind.annotation.PatchMapping;

@PatchMapping("/change/student/name/{studentId}")
public ResponseEntity<StudentResponseDto> changeStudentName(
    @PathVariable String studentId,
    @RequestBody StudentRequestDto requestDto
) {
    return ResponseEntity.ok(studentService.updateStudentName(studentId, requestDto));
}

@Transactional
public StudentResponseDto updateStudentName(String studentId, StudentRequestDto requestDto) {
    Student findStudent = studentRepository.findByStudentId(studentId)
        .orElseThrow(() -> new IllegalArgumentException("존재하지 않는 학생입니다."));

    findStudent.updateStudentName(requestDto.getName());

    // 불필요한 save 호출
    Student savedStudent = studentRepository.save(findStudent);

    return StudentResponseDto.fromEntity(savedStudent);
}

@Transactional
public StudentResponseDto updateStudentName(String studentId, StudentRequestDto requestDto) {
    // 1. 엔티티 조회 → 영속성 컨텍스트가 관리 시작
    Student findStudent = studentRepository.findByStudentId(studentId)
        .orElseThrow(() -> new IllegalArgumentException("존재하지 않는 학생입니다."));

    // 2. 엔티티 상태 변경
    findStudent.updateStudentName(requestDto.getName());

    // 3. save() 없이도 트랜잭션 커밋 시 자동으로 UPDATE 쿼리 실행
    return StudentResponseDto.fromEntity(findStudent);
}