@Transient 필드가 10개가 되었을 때 -- 엔티티에서 DTO로 분리한 과정

@Entity
public class Building {
    @Id
    private Long id;
    private String name;
    private String address;
 
    @Transient
    private List<String> availableLectureRooms;
}

@Entity
public class Building {
    @Id
    private Long id;
    private String name;
 
    // (1) javax.persistence.Transient -- DB 제외, JSON에는 나감
    @javax.persistence.Transient
    private double distanceFromUser;
 
    // (2) java.beans.Transient -- JavaBeans 스펙. Jackson 2.6+에서 직렬화 제외
    @java.beans.Transient
    public String getInternalCode() {
        return "INTERNAL-" + id;
    }
 
    // (3) transient 키워드 -- DB 제외, Jackson 기본 설정에서도 제외
    private transient String tempCalculation;
}

@RestController
public class BuildingController {
    @GetMapping("/buildings/{id}")
    public Building getBuilding(@PathVariable Long id) {
        Building b = buildingRepository.findById(id).orElseThrow();
        b.setDistanceFromUser(3.5); // 임시 계산값
        return b; // JSON에 distanceFromUser: 3.5 가 그대로 나감
    }
}

@Entity
public class Building {
    @Id
    private Long id;
 
    @ManyToOne(fetch = FetchType.LAZY)
    private Region region; // 프록시 상태로 로딩
}
 
@GetMapping("/buildings/{id}")
public Building getBuilding(@PathVariable Long id) {
    Building b = buildingRepository.findById(id).orElseThrow();
    return b;
    // Jackson이 b.getRegion()을 호출하는 순간:
    // org.hibernate.LazyInitializationException:
    //   could not initialize proxy [Region#1] - no Session
}

@Entity
public class Building {
    @OneToMany(mappedBy = "building", fetch = FetchType.LAZY)
    private List<Floor> floors;
}
 
@GetMapping("/buildings")
public List<Building> list() {
    List<Building> buildings = buildingRepository.findAll();
    return buildings;
    // Jackson이 각 building.getFloors() 호출
    // SELECT * FROM building                       -- 1번
    // SELECT * FROM floor WHERE building_id = 1    -- N번
    // SELECT * FROM floor WHERE building_id = 2
    // ...
}

// 엔티티 -- DB 매핑만 담당
@Entity
public class Building {
    @Id
    private Long id;
    private String name;
    private String address;
 
    @ManyToOne(fetch = FetchType.LAZY)
    private Region region;
 
    @OneToMany(mappedBy = "building", fetch = FetchType.LAZY)
    private List<Floor> floors;
}
 
// 응답 DTO -- 프레젠테이션 관심사 담당
public record BuildingResponseDto(
    Long id,
    String name,
    String address,
    String regionName,
    List<String> availableLectureRooms
) {
    public static BuildingResponseDto from(Building building, List<String> rooms) {
        return new BuildingResponseDto(
            building.getId(),
            building.getName(),
            building.getAddress(),
            building.getRegion().getName(),
            rooms
        );
    }
}

public record BuildingResponseDto(Long id, String name, String regionName) {
    public static BuildingResponseDto from(Building building) {
        return new BuildingResponseDto(
            building.getId(),
            building.getName(),
            building.getRegion().getName()
        );
    }
}

@Query("SELECT new com.example.dto.BuildingListDto(b.id, b.name, b.region.name) "
     + "FROM Building b")
List<BuildingListDto> findAllAsDto();

// 인터페이스 프로젝션 -- 구현체를 Spring이 프록시로 생성
public interface BuildingListView {
    Long getId();
    String getName();
    String getRegionName();
}
 
public interface BuildingRepository extends JpaRepository<Building, Long> {
    List<BuildingListView> findAllBy(); // 반환 타입만 바꾸면 됨
}

// 이러면 엔티티 전체를 로딩함 -- SELECT 최적화 없음
public interface BuildingListView {
    @Value("#{target.name + ' (' + target.region.name + ')'}")
    String getDisplayName();
}

@Mapper(componentModel = "spring")
public interface BuildingMapper {
    BuildingResponseDto toDto(Building building);
}

@Mapper(componentModel = "spring")
public interface BuildingMapper {
 
    @Mapping(target = "statusLabel", ignore = true)
    BuildingDetailDto toDetailDto(Building building);
 
    @AfterMapping
    default void enrichDetailDto(Building source, @MappingTarget BuildingDetailDto target) {
        if (source.getStatus() == BuildingStatus.ACTIVE
                && source.getFloors().size() > 10) {
            target.setStatusLabel("대형 운영중");
        } else if (source.getStatus() == BuildingStatus.ACTIVE) {
            target.setStatusLabel("운영중");
        } else {
            target.setStatusLabel("비활성");
        }
    }
}

// 목록 조회 -- 최소 정보만
public record BuildingListDto(Long id, String name, String regionName) {}
 
// 상세 조회 -- 연관 데이터 포함
public record BuildingDetailDto(
    Long id, String name, String address,
    RegionDto region, List<FloorDto> floors,
    LocalDateTime createdAt
) {}
 
// 관리자 전용 -- 내부 메타 포함
public record BuildingAdminDto(
    Long id, String name, String internalCode,
    BigDecimal monthlyRevenue,
    List<AuditLogDto> recentAuditLogs
) {}

@RestController
@RequestMapping("/buildings")
public class BuildingController {
 
    @GetMapping
    public List<BuildingListDto> list() {
        return buildingQueryService.findAllAsList(); // join 없이 단일 테이블
    }
 
    @GetMapping("/{id}")
    public BuildingDetailDto detail(@PathVariable Long id) {
        return buildingQueryService.findDetail(id); // fetch join
    }
 
    @GetMapping("/{id}/admin")
    @PreAuthorize("hasRole('ADMIN')")
    public BuildingAdminDto adminDetail(@PathVariable Long id) {
        return buildingQueryService.findAdminDetail(id); // 감사로그 join
    }
}

@Entity
public class Building {
    @Id
    private Long id;
    private String name;
 
    // @Transient: Hibernate가 완전히 무시. dirty check 대상 아님.
    @Transient
    private double distanceFromUser;
 
    // @Formula: DB에서 서브쿼리로 계산. 읽기 전용.
    @Formula("(SELECT COUNT(*) FROM floor f WHERE f.building_id = id)")
    private int floorCount;
}