영속성 컨텍스트 고려하자

@Entity
@Getter
public class Member {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "member_id")
    private Long id;
	
    ... 
    
    @JsonIgnore
    @OneToMany(mappedBy = "member")
    private List<Interest> interests = new ArrayList<>();

}


@Entity
@Getter @Setter
public class Interest {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "interest_id")
    private Long id;
    @ManyToOne
    @JoinColumn(name = "member_id")
    private Member member;
    @JsonIgnore
    @ManyToOne
    @JoinColumn(name = "attraction_id")
    private Attraction attraction;

}

@Service
@Transactional(readOnly = true)
@RequiredArgsConstructor
public class InterestService {
    private final InterestRepository interestRepository;
    private final MemberRepository memberRepository;
    private final AttractionRepository attractionRepository;



    @Transactional(readOnly = false)
    public void registerInterest(Long memberId, List<Long> attractionIds) {
        Member member = memberRepository.findById(memberId)
                .orElseThrow(() -> new IllegalArgumentException("유효하지 않은 멤버 ID입니다"));

        // 해당 회원 관심 지역 비우기
        List<Interest> interests = member.getInterests();
        for (Interest interest : interests) {
            interestRepository.delete(interest);
        }

        // 새로운 관심 지역 등록
        for (Long attractionId : attractionIds) {
            Attraction attraction = attractionRepository.findById(attractionId)
                    .orElseThrow(() -> new IllegalArgumentException("Invalid attraction id"));
            Interest interest = new Interest();
            interest.setMember(member);
            interest.setAttraction(attraction);
            interestRepository.save(interest);
        }

    }

}
DB에서는 지워지지만 영속성 컨텍스트에서는 반영이 안됨. 

영속성 컨텍스트 반영을 위해 줬다

@Service
@Transactional(readOnly = true)
@RequiredArgsConstructor
public class InterestService {
    private final InterestRepository interestRepository;
    private final MemberRepository memberRepository;
    private final AttractionRepository attractionRepository;



    @Transactional(readOnly = false)
    public void registerInterest(Long memberId, List<Long> attractionIds) {
        Member member = memberRepository.findById(memberId)
                .orElseThrow(() -> new IllegalArgumentException("유효하지 않은 멤버 ID입니다"));

        // 해당 회원 관심 지역 비우기
        List<Interest> interests = member.getInterests();
        for (Interest interest : interests) {
            interestRepository.delete(interest);
        }

        // 영속성 컨텍스트에 반영
        member.clearInterest();

        // 새로운 관심 지역 등록
        for (Long attractionId : attractionIds) {
            Attraction attraction = attractionRepository.findById(attractionId)
                    .orElseThrow(() -> new IllegalArgumentException("Invalid attraction id"));
            Interest interest = new Interest();
            interest.setMember(member);
            interest.setAttraction(attraction);
            interestRepository.save(interest);
            member.getInterests().add(interest);
        }
    }


}
@Entity
@Getter
public class Member {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "member_id")
    private Long id;

 	... 
    
    @JsonIgnore
    @OneToMany(mappedBy = "member")
    private List<Interest> interests = new ArrayList<>();

    @OneToMany(mappedBy = "member")
    private List<Stamp> stamps = new ArrayList<>();

    // 영속성 컨텍스트 비워주는 비지니스 메서드
    public void clearInterest(){
        this.interests = new ArrayList<>();
    }

DB 반영 이후 영속성을 고려하지 않으면 후에 .. 대단한 장애를 보게 된다.