[Fix] 휴지통 폴더 목록 materialCount가 항상 0으로 나오던 문제 수정

src/main/java/com/teaching/backend/domain/folder/entity/Folder.java

@@ -26,6 +26,11 @@ public class Folder extends BaseSoftDeleteEntity {
     @Column(nullable = false, length = 100)
     private String name;
 
+    /**
+     * DB 컬럼(item_count)이 NOT NULL이고 기본값이 없어 필드 자체는 유지해야 하지만,
+     * 값은 어디에서도 갱신되지 않아 항상 0으로 고정된 죽은 필드다 — 읽지 말 것
+     * (휴지통 폴더의 자료 개수는 TrashService 에서 native COUNT 쿼리로 별도 계산한다).
+     */
     @Column(nullable = false)
     @Builder.Default
     private Integer itemCount = 0;

src/main/java/com/teaching/backend/domain/material/repository/MaterialRepository.java

@@ -287,6 +287,23 @@ List<FolderMaterialRestoreCountProjection> countDeletedByFolderIdForRestore(
             @Param("userId") Long userId
     );
 
+    /** 휴지통 폴더 목록 조회용: 폴더와 함께 삭제된(휴지통으로 같이 이동한) 자료 개수를 폴더별로 집계한다. */
+    @Query(
+            value = """
+                    SELECT folder_id AS folderId, COUNT(*) AS materialCount
+                    FROM materials
+                    WHERE folder_id IN (:folderIds)
+                      AND user_id = :userId
+                      AND deleted_at IS NOT NULL
+                    GROUP BY folder_id
+                    """,
+            nativeQuery = true
+    )
+    List<FolderMaterialRestoreCountProjection> countDeletedByFolderIdsAndUserId(
+            @Param("folderIds") List<Long> folderIds,
+            @Param("userId") Long userId
+    );
+
     /** 휴지통에 있는 폴더 상세 조회용: 그 폴더 안에 있던(폴더와 함께 휴지통으로 이동한) 자료를 최신순으로 조회한다. */
     @Query(
             value = """

src/main/java/com/teaching/backend/domain/trash/dto/response/TrashFolderItemResponse.java

@@ -11,18 +11,12 @@ public record TrashFolderItemResponse(
         LocalDateTime deletedAt
 ) {
 
-    public static TrashFolderItemResponse from(Folder folder) {
+    public static TrashFolderItemResponse from(Folder folder, long materialCount) {
         return new TrashFolderItemResponse(
                 folder.getId(),
                 folder.getName(),
-                convertToLong(folder.getItemCount()),
+                materialCount,
                 folder.getDeletedAt()
         );
     }
-
-    private static Long convertToLong(Integer itemCount) {
-        return itemCount == null
-                ? 0L
-                : itemCount.longValue();
-    }
 }

src/main/java/com/teaching/backend/domain/trash/service/TrashService.java

@@ -87,7 +87,23 @@ public TrashFolderListResponse getTrashedFolders(Long userId, String sort, Integ
                 ? folderRepository.findTrashedByUserIdOrderByDeletedAtAsc(userId, pageRequest)
                 : folderRepository.findTrashedByUserIdOrderByDeletedAtDesc(userId, pageRequest);
 
-        return TrashFolderListResponse.of(folders.map(TrashFolderItemResponse::from));
+        Map<Long, Long> materialCountByFolderId = getDeletedMaterialCountByFolderId(folders.getContent(), userId);
+
+        return TrashFolderListResponse.of(folders.map(folder ->
+                TrashFolderItemResponse.from(folder, materialCountByFolderId.getOrDefault(folder.getId(), 0L))
+        ));
+    }
+
+    private Map<Long, Long> getDeletedMaterialCountByFolderId(List<Folder> folders, Long userId) {
+        if (folders.isEmpty()) {
+            return Map.of();
+        }
+        List<Long> folderIds = folders.stream().map(Folder::getId).toList();
+        return materialRepository.countDeletedByFolderIdsAndUserId(folderIds, userId).stream()
+                .collect(Collectors.toMap(
+                        FolderMaterialRestoreCountProjection::getFolderId,
+                        FolderMaterialRestoreCountProjection::getMaterialCount
+                ));
     }
 
     public TrashMaterialListResponse getTrashedMaterials(Long userId, String sort, Integer page) {

src/test/java/com/teaching/backend/domain/material/repository/MaterialRepositoryTest.java

@@ -212,6 +212,39 @@ void findFolderNamesByIdsExcludesMaterialsOwnedByOtherUsers() {
         assertThat(result).isEmpty();
     }
 
+    /** 휴지통 폴더 목록의 materialCount 계산에 쓰이는 쿼리 — 삭제된 자료만, 요청한 유저 소유만, 폴더별로 정확히 집계되는지 검증. */
+    @Test
+    void countDeletedByFolderIdsAndUserIdGroupsDeletedMaterialsPerFolderScopedToOwner() {
+        User owner = userRepository.save(user("trash-count-owner"));
+        User other = userRepository.save(user("trash-count-other"));
+        Folder folderWithTwoDeleted = folderRepository.save(Folder.create(owner, "Folder X"));
+        Folder folderWithOneDeleted = folderRepository.save(Folder.create(owner, "Folder Y"));
+        Folder otherUsersFolder = folderRepository.save(Folder.create(other, "Folder Z"));
+
+        Material activeInFolderX = materialRepository.save(material(owner, folderWithTwoDeleted, "https://example.com/active"));
+        Material deletedInFolderX1 = materialRepository.save(material(owner, folderWithTwoDeleted, "https://example.com/x1"));
+        Material deletedInFolderX2 = materialRepository.save(material(owner, folderWithTwoDeleted, "https://example.com/x2"));
+        Material deletedInFolderY = materialRepository.save(material(owner, folderWithOneDeleted, "https://example.com/y"));
+        Material deletedInOthersFolder = materialRepository.save(material(other, otherUsersFolder, "https://example.com/z"));
+        deletedInFolderX1.delete();
+        deletedInFolderX2.delete();
+        deletedInFolderY.delete();
+        deletedInOthersFolder.delete();
+        flushAndClear();
+
+        List<FolderMaterialRestoreCountProjection> result = materialRepository.countDeletedByFolderIdsAndUserId(
+                List.of(folderWithTwoDeleted.getId(), folderWithOneDeleted.getId(), otherUsersFolder.getId()),
+                owner.getId()
+        );
+
+        assertThat(result)
+                .extracting(FolderMaterialRestoreCountProjection::getFolderId, FolderMaterialRestoreCountProjection::getMaterialCount)
+                .containsExactlyInAnyOrder(
+                        tuple(folderWithTwoDeleted.getId(), 2L),
+                        tuple(folderWithOneDeleted.getId(), 1L)
+                );
+    }
+
     private User user(String suffix) {
         return User.create(
                 "material-repository-" + suffix + "@example.com",

src/test/java/com/teaching/backend/domain/trash/service/TrashServiceTest.java

@@ -20,6 +20,7 @@
 import com.teaching.backend.domain.tag.repository.MaterialTagRepository;
 import com.teaching.backend.domain.teachingmap.repository.TeachingMapRepository;
 import com.teaching.backend.domain.teachingmap.repository.TeachingMapStepRepository;
+import com.teaching.backend.domain.trash.dto.response.TrashFolderListResponse;
 import com.teaching.backend.domain.trash.dto.response.TrashFolderMaterialListResponse;
 import com.teaching.backend.domain.trash.dto.response.TrashMaterialItemResponse;
 import com.teaching.backend.domain.trash.dto.response.TrashMaterialListResponse;
@@ -140,6 +141,46 @@ void queriesTagRepositoryExactlyOnceForNonEmptyPage() {
         assertThat(materialIdsCaptor.getValue()).containsExactlyInAnyOrder(10L, 20L);
     }
 
+    /**
+     * 휴지통 폴더 목록의 materialCount는 폴더 엔티티의 죽은 itemCount 필드가 아니라,
+     * 삭제된 자료를 직접 COUNT한 값이어야 한다 (never-updated itemCount=0 회귀 방지).
+     */
+    @Test
+    void trashedFolderListUsesLiveDeletedMaterialCountNotStaleItemCountField() {
+        Folder folder = folder(FOLDER_ID, "테스트테테");
+        when(folderRepository.findTrashedByUserIdOrderByDeletedAtDesc(any(), any()))
+                .thenReturn(new PageImpl<>(List.of(folder)));
+        when(materialRepository.countDeletedByFolderIdsAndUserId(List.of(FOLDER_ID), USER_ID))
+                .thenReturn(List.of(restoreCount(FOLDER_ID, 6L)));
+
+        TrashFolderListResponse response = trashService.getTrashedFolders(USER_ID, null, 0);
+
+        assertThat(response.content()).extracting("materialCount").containsExactly(6L);
+    }
+
+    @Test
+    void trashedFolderListDefaultsToZeroWhenNoDeletedMaterialsMatchFolder() {
+        Folder folder = folder(FOLDER_ID, "빈폴더");
+        when(folderRepository.findTrashedByUserIdOrderByDeletedAtDesc(any(), any()))
+                .thenReturn(new PageImpl<>(List.of(folder)));
+        when(materialRepository.countDeletedByFolderIdsAndUserId(List.of(FOLDER_ID), USER_ID))
+                .thenReturn(List.of());
+
+        TrashFolderListResponse response = trashService.getTrashedFolders(USER_ID, null, 0);
+
+        assertThat(response.content()).extracting("materialCount").containsExactly(0L);
+    }
+
+    @Test
+    void trashedFolderListDoesNotQueryMaterialCountsForEmptyPage() {
+        when(folderRepository.findTrashedByUserIdOrderByDeletedAtDesc(any(), any()))
+                .thenReturn(new PageImpl<>(List.of()));
+
+        trashService.getTrashedFolders(USER_ID, null, 0);
+
+        verify(materialRepository, never()).countDeletedByFolderIdsAndUserId(anyList(), any());
+    }
+
     @Test
     void restoreFoldersCascadesMaterialRestoreOnlyForActuallyRestoredFolders() {
         when(folderRepository.restoreTrashedFolderIfNameAvailable(10L, USER_ID)).thenReturn(1);