프로젝션과 결과 반환 - DTO 조회

순수 JPA에서 DTO 조회

MemberDto

@Data
public class MemberDto {

    private String username;
    private int age;

    public MemberDto(String username, int age) {
        this.username = username;
        this.age = age;
    }
}

 

순수 JPA에서 DTO 조회 코드

@Test
    public void findDtoByJPQL() {
        List<MemberDto> result = em.createQuery("select new study.querydsl.dto.MemberDto(m.username, m.age) from Member m", MemberDto.class)
                .getResultList();

        for (MemberDto memberDto : result) {
            System.out.println("memberDto = " + memberDto);
        }
    }

순수 JPA에서 DTO를 조회할 때는 new 명령어를 사용해야함
DTO의 package이름을 다 적어줘야해서 지저분함
생성자 방식만 지원함

 

 

 

Querydsl 빈 생성(Bean population)

결과를 DTO 반환할 때 사용
다음 3가지 방법 지원

 

프로퍼티 접근
필드 직접 접근
생성자 사용

 

 

프로퍼티 접근 - Setter

@Test
    public void findDtoBySetter() {
        List<MemberDto> result = queryFactory
                .select(Projections.bean(MemberDto.class,
                        member.username,
                        member.age))
                .from(member)
                .fetch();

        for (MemberDto memberDto : result) {
            System.out.println("memberDto = " + memberDto);
        }
    }

 

 

 

필드 직접 접근

@Test
    public void findDtoByField() {
        List<MemberDto> result = queryFactory
                .select(Projections.fields(MemberDto.class,
                        member.username,
                        member.age))
                .from(member)
                .fetch();

        for (MemberDto memberDto : result) {
            System.out.println("memberDto = " + memberDto);
        }
    }

 

 

 

 

별칭이 다를 때

package study.querydsl.dto;
import lombok.Data;
@Data
public class UserDto {
 private String name;
 private int age;
}



List<UserDto> fetch = queryFactory
 .select(Projections.fields(UserDto.class,
 member.username.as("name"),
 ExpressionUtils.as(
 JPAExpressions
 .select(memberSub.age.max())
 .from(memberSub), "age")
 )
 ).from(member)
 .fetch();

프로퍼티나, 필드 접근 생성 방식에서 이름이 다를 때 해결 방안
ExpressionUtils.as(source,alias) : 필드나, 서브 쿼리에 별칭 적용
username.as("memberName") : 필드에 별칭 적용

 

 

 

 

 

생성자 사용

@Test
public void findDtoByConstructor() {
    List<MemberDto> result = queryFactory
            .select(Projections.constructor(MemberDto.class,
                    member.username,
                    member.age))
            .from(member)
            .fetch();

    for (MemberDto memberDto : result) {
        System.out.println("memberDto = " + memberDto);
    }
}

 

 

 

 

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