Ref: Unique Key 충돌 및 flush 타이밍 문제 해결 (#263)

Author: joyewon0705

src/main/java/com/back/domain/board/post/entity/Post.java

@@ -62,6 +62,10 @@ public void addPostCategoryMapping(PostCategoryMapping mapping) {
         this.postCategoryMappings.add(mapping);
     }
 
+    public void removePostCategoryMapping(PostCategoryMapping mapping) {
+        this.postCategoryMappings.remove(mapping);
+    }
+
     public void addLike(PostLike like) {
         this.postLikes.add(like);
     }
@@ -93,11 +97,23 @@ public void update(String title, String content) {
         this.content = content;
     }
 
-    // TODO: 진짜로 바뀐 카테고리만 추가/삭제하도록 개선
     /** 카테고리 일괄 업데이트 */
-    public void updateCategories(List<PostCategory> categories) {
-        this.postCategoryMappings.clear();
-        categories.forEach(category -> new PostCategoryMapping(this, category));
+    public void updateCategories(List<PostCategory> newCategories) {
+        List<PostCategory> currentCategories = this.getCategories();
+
+        // 제거 대상
+        List<PostCategoryMapping> toRemove = this.getPostCategoryMappings().stream()
+                .filter(mapping -> !newCategories.contains(mapping.getCategory()))
+                .toList();
+
+        // 추가 대상
+        List<PostCategory> toAdd = newCategories.stream()
+                .filter(category -> !currentCategories.contains(category))
+                .toList();
+
+        // 실행
+        toRemove.forEach(this::removePostCategoryMapping);
+        toAdd.forEach(category -> new PostCategoryMapping(this, category));
     }
 
     /** 좋아요 수 증가 */

src/main/java/com/back/domain/board/post/service/PostService.java

@@ -49,17 +49,18 @@ public PostResponse createPost(PostRequest request, Long userId) {
         // Post 생성
         Post post = new Post(user, request.title(), request.content(), request.thumbnailUrl());
 
+        // Post 저장
+        Post saved = postRepository.save(post);
+
         // Category 매핑
         if (request.categoryIds() != null) {
             List<PostCategory> categories = postCategoryRepository.findAllById(request.categoryIds());
             if (categories.size() != request.categoryIds().size()) {
                 throw new CustomException(ErrorCode.CATEGORY_NOT_FOUND);
             }
-            post.updateCategories(categories);
+            saved.updateCategories(categories);
         }
 
-        // Post 저장 및 응답 반환
-        Post saved = postRepository.save(post);
         return PostResponse.from(saved);
     }