QueryDSL — 타입 안전한 동적 쿼리를 작성하는 방법

// 문자열 JPQL — 오타가 있어도 컴파일은 통과
em.createQuery("SELECT m FROM Membr WHERE m.age > :age"); // Membr 오타!

// QueryDSL — 컴파일 시점에 오류 감지
queryFactory
    .selectFrom(member)
    .where(member.age.gt(20))
    .fetch();

// build.gradle
dependencies {
    // OpenFeign 포크 — Jakarta EE 지원
    implementation 'io.github.openfeign.querydsl:querydsl-jpa:6.x'
    annotationProcessor 'io.github.openfeign.querydsl:querydsl-apt:6.x:jakarta'
    annotationProcessor 'jakarta.annotation:jakarta.annotation-api'
    annotationProcessor 'jakarta.persistence:jakarta.persistence-api'
}

@Configuration
public class QueryDslConfig {

    @Bean
    public JPAQueryFactory jpaQueryFactory(EntityManager em) {
        return new JPAQueryFactory(em);
    }
}

@RequiredArgsConstructor
@Repository
public class MemberQueryRepository {

    private final JPAQueryFactory queryFactory;

    public List<Member> findByAge(int age) {
        QMember member = QMember.member;

        return queryFactory
            .selectFrom(member)
            .where(member.age.eq(age))
            .orderBy(member.name.asc())
            .fetch();
    }
}

// 내부 조인
queryFactory
    .selectFrom(member)
    .join(member.team, team)
    .where(team.name.eq("개발팀"))
    .fetch();

// 왼쪽 외부 조인 + on 절
queryFactory
    .selectFrom(member)
    .leftJoin(member.team, team)
    .on(team.active.isTrue())
    .fetch();

// 생성자 방식
queryFactory
    .select(Projections.constructor(MemberDto.class,
        member.name,
        member.age))
    .from(member)
    .fetch();

// @QueryProjection 방식 — DTO 생성자에 어노테이션 추가
queryFactory
    .select(new QMemberDto(member.name, member.age))
    .from(member)
    .fetch();

public List<Member> search(String name, Integer minAge, Integer maxAge) {
    QMember member = QMember.member;
    BooleanBuilder builder = new BooleanBuilder();

    if (name != null) {
        builder.and(member.name.contains(name));
    }
    if (minAge != null) {
        builder.and(member.age.goe(minAge));
    }
    if (maxAge != null) {
        builder.and(member.age.loe(maxAge));
    }

    return queryFactory
        .selectFrom(member)
        .where(builder)
        .fetch();
}

public List<Member> search(String name, Integer minAge, Integer maxAge) {
    return queryFactory
        .selectFrom(member)
        .where(
            nameContains(name),
            ageGoe(minAge),
            ageLoe(maxAge)
        )
        .fetch();
}

// 각 조건을 별도 메서드로 분리 — 재사용 가능
private BooleanExpression nameContains(String name) {
    return name != null ? member.name.contains(name) : null;
}

private BooleanExpression ageGoe(Integer minAge) {
    return minAge != null ? member.age.goe(minAge) : null;
}

private BooleanExpression ageLoe(Integer maxAge) {
    return maxAge != null ? member.age.loe(maxAge) : null;
}

public interface MemberRepository extends JpaRepository<Member, Long>,
                                          QuerydslPredicateExecutor<Member> {
}

// 사용
QMember member = QMember.member;
Predicate predicate = member.age.gt(20).and(member.name.contains("김"));
Iterable<Member> result = memberRepository.findAll(predicate);

// 1. 커스텀 인터페이스 정의
public interface MemberRepositoryCustom {
    List<MemberDto> searchMembers(MemberSearchCondition condition);
}

// 2. 구현 클래스 — 반드시 Impl 접미사
@RequiredArgsConstructor
public class MemberRepositoryImpl implements MemberRepositoryCustom {

    private final JPAQueryFactory queryFactory;

    @Override
    public List<MemberDto> searchMembers(MemberSearchCondition condition) {
        return queryFactory
            .select(new QMemberDto(
                member.name,
                member.age,
                team.name))
            .from(member)
            .leftJoin(member.team, team)
            .where(
                nameContains(condition.getName()),
                ageGoe(condition.getMinAge()),
                teamNameEq(condition.getTeamName())
            )
            .fetch();
    }
}

// 3. 기존 Repository에 상속 추가
public interface MemberRepository extends JpaRepository<Member, Long>,
                                          MemberRepositoryCustom {
}

public Page<MemberDto> searchPage(MemberSearchCondition condition, Pageable pageable) {
    List<MemberDto> content = queryFactory
        .select(new QMemberDto(member.name, member.age))
        .from(member)
        .where(nameContains(condition.getName()))
        .offset(pageable.getOffset())
        .limit(pageable.getPageSize())
        .orderBy(member.id.desc())
        .fetch();

    // count 쿼리 — 조인이 필요 없으면 단순화 가능
    JPAQuery<Long> countQuery = queryFactory
        .select(member.count())
        .from(member)
        .where(nameContains(condition.getName()));

    // count 쿼리 최적화 — 마지막 페이지이거나 첫 페이지에서 전체가 다 나오면 count 생략
    return PageableExecutionUtils.getPage(content, pageable, countQuery::fetchOne);
}

// 기존 — 문자열 기반 (오타 위험)
Sort.by("name");

// 새로운 방식 — 메서드 레퍼런스 기반 (컴파일 시점 검증)
Sort.by(TypedSort.sort(Member.class).by(Member::getName));