[Refactor/OPS-380] 스페이스 목록 조회 개선 (#156)

* refactor : membership 조회 시 lazy loading에 의한 n+1 문제 개선

* refactor : space의 member 목록 조회 시 n+ 1 문제 해결

---------

Co-authored-by: EpicFn <rlagurwls056@gmail.com>

Author: EpicFn

src/main/java/org/tuna/zoopzoop/backend/domain/space/membership/repository/MembershipRepository.java

@@ -4,6 +4,7 @@
 import org.springframework.data.domain.Pageable;
 import org.springframework.data.jpa.repository.JpaRepository;
 import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
 import org.tuna.zoopzoop.backend.domain.member.entity.Member;
 import org.tuna.zoopzoop.backend.domain.space.membership.entity.Membership;
 import org.tuna.zoopzoop.backend.domain.space.membership.enums.Authority;
@@ -45,4 +46,17 @@ public interface MembershipRepository extends JpaRepository<Membership, Integer>
     where m.member.id = :memberId and m.space.id = :spaceId
 """)
     Optional<Membership> findByMemberIdAndSpaceId(Integer memberId, Integer spaceId);
+
+    @Query("SELECT m FROM Membership m JOIN FETCH m.space WHERE m.member = :member ORDER BY m.id ASC")
+    Page<Membership> findAllByMemberWithSpace(@Param("member") Member member, Pageable pageable);
+
+    @Query("SELECT m FROM Membership m JOIN FETCH m.space WHERE m.member = :member AND m.authority = :authority ORDER BY m.id ASC")
+    Page<Membership> findAllByMemberAndAuthorityWithSpace(@Param("member") Member member, @Param("authority") Authority authority, Pageable pageable);
+
+    @Query("SELECT m FROM Membership m JOIN FETCH m.space WHERE m.member = :member AND m.authority <> :authority ORDER BY m.id ASC")
+    Page<Membership> findAllByMemberAndAuthorityIsNotWithSpace(@Param("member") Member member, @Param("authority") Authority authority, Pageable pageable);
+
+    // 여러 Space에 속한 Member 목록 한번에 조회 (JOIN FETCH로 Member 정보까지)
+    @Query("SELECT m FROM Membership m JOIN FETCH m.member WHERE m.space IN :spaces AND m.authority <> org.tuna.zoopzoop.backend.domain.space.membership.enums.Authority.PENDING")
+    List<Membership> findAllMembersInSpaces(@Param("spaces") List<Space> spaces);
 }

src/main/java/org/tuna/zoopzoop/backend/domain/space/membership/service/MembershipService.java

@@ -12,6 +12,7 @@
 import org.tuna.zoopzoop.backend.domain.SSE.service.EmitterService;
 import org.tuna.zoopzoop.backend.domain.member.entity.Member;
 import org.tuna.zoopzoop.backend.domain.member.service.MemberService;
+import org.tuna.zoopzoop.backend.domain.space.membership.dto.etc.SpaceMemberInfo;
 import org.tuna.zoopzoop.backend.domain.space.membership.entity.Membership;
 import org.tuna.zoopzoop.backend.domain.space.membership.enums.Authority;
 import org.tuna.zoopzoop.backend.domain.space.membership.enums.JoinState;
@@ -21,8 +22,11 @@
 import org.tuna.zoopzoop.backend.global.rsData.RsData;
 
 import java.nio.file.AccessDeniedException;
+import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import java.util.Optional;
+import java.util.stream.Collectors;
 
 @Service
 @RequiredArgsConstructor
@@ -64,11 +68,11 @@ public Membership findByMemberAndSpace(Member member, Space space) {
      */
     public Page<Membership> findByMember(Member member, String state, Pageable pageable) {
         if (state.equalsIgnoreCase("PENDING")) {
-            return membershipRepository.findAllByMemberAndAuthorityOrderById(member, Authority.PENDING, pageable);
+            return membershipRepository.findAllByMemberAndAuthorityWithSpace(member, Authority.PENDING, pageable);
         } else if (state.equalsIgnoreCase("JOINED")) {
-            return membershipRepository.findAllByMemberAndAuthorityIsNotOrderById(member, Authority.PENDING, pageable);
+            return membershipRepository.findAllByMemberAndAuthorityIsNotWithSpace(member, Authority.PENDING, pageable);
         } else {
-            return membershipRepository.findAllByMemberOrderById(member, pageable);
+            return membershipRepository.findAllByMemberWithSpace(member, pageable);
         }
     }
 
@@ -107,6 +111,36 @@ public List<Membership> findMembersBySpace(Space space) {
         return membershipRepository.findAllBySpaceAndAuthorityIsNotOrderById(space, Authority.PENDING);
     }
 
+    /**
+     * 여러 스페이스에 속한 멤버 목록을 한 번의 쿼리로 조회 (N+1 문제 해결용)
+     * @param spaces 조회할 스페이스 목록
+     * @return 스페이스 ID를 key로, 해당 스페이스의 멤버 정보 리스트를 value로 갖는 Map
+     */
+    @Transactional(readOnly = true)
+    public Map<Integer, List<SpaceMemberInfo>> findMembersBySpaces(List<Space> spaces) {
+        if (spaces == null || spaces.isEmpty()) {
+            return Collections.emptyMap();
+        }
+
+        // 1. 한 번의 쿼리로 모든 스페이스의 멤버십 정보를 가져옴
+        List<Membership> allMemberships = membershipRepository.findAllMembersInSpaces(spaces);
+
+        // 2. Space ID 별로 그룹핑하여 Map으로 변환
+        return allMemberships.stream()
+                .collect(Collectors.groupingBy(
+                        membership -> membership.getSpace().getId(), // Key: Space ID
+                        Collectors.mapping( // Value: List<SpaceMemberInfo> DTO로 변환
+                                membership -> new SpaceMemberInfo(
+                                        membership.getMember().getId(),
+                                        membership.getMember().getName(),
+                                        membership.getMember().getProfileImageUrl(),
+                                        membership.getAuthority()
+                                ),
+                                Collectors.toList()
+                        )
+                ));
+    }
+
     // ======================== 권한 조회 ======================== //
     /**
      * 멤버가 스페이스의 어드민 권한을 가지고 있는지 확인

src/main/java/org/tuna/zoopzoop/backend/domain/space/space/controller/ApiV1SpaceController.java

@@ -32,7 +32,9 @@
 import org.tuna.zoopzoop.backend.global.security.jwt.CustomUserDetails;
 
 import java.nio.file.AccessDeniedException;
+import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import java.util.stream.Collectors;
 
 @RestController
@@ -144,35 +146,39 @@ public RsData<ResBodyForSpaceListPage> getAllSpaces(
         String stateStr = (state == null) ? "ALL" : state.name();
         Page<Membership> membershipsPage = membershipService.findByMember(member, stateStr, pageable);
 
+        Map<Integer, List<SpaceMemberInfo>> membersBySpaceId = Collections.emptyMap();
+        if (includeMembers) {
+            // 현재 페이지에 포함된 Space 엔티티 목록 추출
+            List<Space> spacesOnPage = membershipsPage.getContent().stream()
+                    .map(Membership::getSpace)
+                    .distinct()
+                    .collect(Collectors.toList());
+            // 멤버 정보를 한 번의 쿼리로 조회
+            membersBySpaceId = membershipService.findMembersBySpaces(spacesOnPage);
+        }
+
         // Page<Membership>를 Page<SpaceMembershipInfo>로 변환
+        final Map<Integer, List<SpaceMemberInfo>> finalMembersMap = membersBySpaceId;
+
         Page<SpaceInfo> spaceInfosPage = membershipsPage.map(membership -> {
             Space space = membership.getSpace();
-            List<SpaceMemberInfo> memberInfos = null;
-
-            if (includeMembers) {
-                // 스페이스에 속한 멤버 목록 조회 (가입 상태만)
-                List<Membership> spaceMemberships = membershipService.findMembersBySpace(space);
-                // 멤버 목록을 DTO로 변환
-                memberInfos = spaceMemberships.stream()
-                        .map(spaceMembership -> new SpaceMemberInfo(
-                                spaceMembership.getMember().getId(),
-                                spaceMembership.getMember().getName(),
-                                spaceMembership.getMember().getProfileImageUrl(),
-                                spaceMembership.getAuthority()
-                        ))
-                        .collect(Collectors.toList());
+
+            // Map에서 spaceId로 멤버 목록을 O(1)에 조회 (기존 N+1 유발 코드 대체)
+            List<SpaceMemberInfo> memberInfos = finalMembersMap.get(space.getId());
+
+            if (includeMembers && memberInfos == null) {
+                memberInfos = Collections.emptyList();
             }
 
             return new SpaceInfo(
                     space.getId(),
                     space.getName(),
                     space.getThumbnailUrl(),
                     membership.getAuthority(),
-                    memberInfos // 조회된 멤버 목록 (null일 수도 있음)
+                    memberInfos
             );
         });
 
-        // 새로운 응답 DTO 생성
         ResBodyForSpaceListPage resBody = new ResBodyForSpaceListPage(spaceInfosPage);
 
         return new RsData<>(