[Fix] 이해도 체크 선택 상태 응답 추가

backend/src/main/java/com/example/Piroin/project/domain/question/dto/QuestionResDTO.java

@@ -145,6 +145,8 @@ public record UnderstandingCheckResponse(
             Integer understoodCount,
             // 오른쪽 X 뱃지 숫자
             Integer notUnderstoodCount,
+            // 현재 로그인 유저가 누른 선택지. 누르지 않았거나 취소한 상태면 null
+            UnderstandResChoice selectedChoice,
             LocalDateTime createdAt
     ) {
     }

backend/src/main/java/com/example/Piroin/project/domain/question/service/QuestionService.java

@@ -59,7 +59,7 @@ public QuestionResDTO.QuestionRoomResponse getQuestionRoom(Long sessionId, int u
         User loginUser = findLoginUser(userId);
         return new QuestionResDTO.QuestionRoomResponse(
                 toSessionResponse(session),
-                getUnderstandingSlice(session, understandingIndex),
+                getUnderstandingSlice(session, understandingIndex, loginUser),
                 getQuestionGroups(session, loginUser)
         );
     }
@@ -543,7 +543,11 @@ private QuestionResDTO.SessionResponse toSessionResponse(StudySession session) {
         );
     }
 
-    private QuestionResDTO.UnderstandingSliceResponse getUnderstandingSlice(StudySession session, int understandingIndex) {
+    private QuestionResDTO.UnderstandingSliceResponse getUnderstandingSlice(
+            StudySession session,
+            int understandingIndex,
+            User loginUser
+    ) {
         Page<UnderstandingCheck> understandingPage = understandingCheckRepository
                 .findBySessionOrderByCreatedAtDesc(session, PageRequest.of(understandingIndex, UNDERSTANDING_PAGE_SIZE));
 
@@ -559,17 +563,17 @@ private QuestionResDTO.UnderstandingSliceResponse getUnderstandingSlice(StudySes
         // attendanceCount는 프론트 화면의 "13/29" 중 29에 해당한다.
         int attendanceCount = attendanceService.countAttendedBySession(session);
         return new QuestionResDTO.UnderstandingSliceResponse(
-                toUnderstandingCheckResponse(current, attendanceCount), understandingIndex, totalCount,
+                toUnderstandingCheckResponse(current, attendanceCount, loginUser), understandingIndex, totalCount,
                 understandingIndex < totalCount - 1, understandingIndex > 0
         );
     }
 
     private QuestionResDTO.UnderstandingCheckResponse toUnderstandingCheckResponse(UnderstandingCheck check) {
-        return toUnderstandingCheckResponse(check, null);
+        return toUnderstandingCheckResponse(check, null, null);
     }
 
     private QuestionResDTO.UnderstandingCheckResponse toUnderstandingCheckResponse(
-            UnderstandingCheck check, Integer attendanceCount
+            UnderstandingCheck check, Integer attendanceCount, User loginUser
     ) {
         // understoodCount/notUnderstoodCount는 오른쪽 O/X 뱃지 숫자로 그대로 사용한다.
         int understoodCount = understandingResponseRepository.countByCheckAndChoice(
@@ -585,10 +589,20 @@ private QuestionResDTO.UnderstandingCheckResponse toUnderstandingCheckResponse(
                 attendanceCount,
                 understoodCount,
                 notUnderstoodCount,
+                getSelectedChoice(check, loginUser),
                 check.getCreatedAt()
         );
     }
 
+    private UnderstandResChoice getSelectedChoice(UnderstandingCheck check, User loginUser) {
+        if (loginUser == null) {
+            return null;
+        }
+        return understandingResponseRepository.findByCheckAndUser(check, loginUser)
+                .map(UnderstandingResponse::getChoice)
+                .orElse(null);
+    }
+
     private QuestionResDTO.QuestionGroupsResponse getQuestionGroups(StudySession session, User loginUser) {
         List<Question> questions = questionRepository.findBySessionAndDeletedAtIsNull(session);
         QuestionSummaryContext summaryContext = getQuestionSummaryContext(questions, loginUser);

frontend/src/pages/qna/QnAListPage.js

@@ -62,6 +62,13 @@ function QnAListPage() {
 
             setSessionTitle(`${session.week}주차 ${DAY_OF_WEEK_KO[session.dayOfWeek]}요일 ${DAY_PART_KO[session.dayPart]} (${session.title})`);
             setUnderstanding(understanding);
+            const currentCheck = understanding?.current;
+            if (currentCheck?.checkId) {
+                setMyChoices(prev => ({
+                    ...prev,
+                    [currentCheck.checkId]: currentCheck.selectedChoice ?? null,
+                }));
+            }
 
             const allQ = [
                 ...(questions.popularQuestions ?? []),
@@ -116,29 +123,31 @@ function QnAListPage() {
     const handleUnderstandChoice = async (choice) => {
         if (!understanding?.current?.checkId) return;
         const checkId = understanding.current.checkId;
-        const newChoice = myChoices[checkId] === choice ? null : choice;
+        const previousChoice = myChoices[checkId] ?? null;
+        const newChoice = previousChoice === choice ? null : choice;
         setMyChoices(prev => ({ ...prev, [checkId]: newChoice }));
-        if (!newChoice) return;
         try {
             const res = await authFetch(
                 `/api/sessions/${sessionId}/understanding-checks/${checkId}/responses`,
-                { method: 'POST', body: JSON.stringify({ choice: newChoice }) }
+                { method: 'POST', body: JSON.stringify({ choice }) }
             );
             if (!res.ok) throw new Error();
             const json = await res.json();
-            if (json.isSuccess) {
-                setUnderstanding(prev => ({
-                    ...prev,
-                    current: {
-                        ...prev.current,
-                        understoodCount: json.result.understoodCount,
-                        notUnderstoodCount: json.result.notUnderstoodCount,
-                        attendanceCount: json.result.attendanceCount,
-                        respondedCount: json.result.respondedCount,
-                    }
-                }));
-            }
+            if (!json.isSuccess) throw new Error(json.message);
+            setMyChoices(prev => ({ ...prev, [checkId]: json.result.selectedChoice ?? null }));
+            setUnderstanding(prev => ({
+                ...prev,
+                current: {
+                    ...prev.current,
+                    understoodCount: json.result.understoodCount,
+                    notUnderstoodCount: json.result.notUnderstoodCount,
+                    attendanceCount: json.result.attendanceCount,
+                    respondedCount: json.result.respondedCount,
+                    selectedChoice: json.result.selectedChoice,
+                }
+            }));
         } catch (err) {
+            setMyChoices(prev => ({ ...prev, [checkId]: previousChoice }));
             console.error('이해도 응답 실패:', err);
         }
     };
@@ -593,4 +602,4 @@ function QnAListPage() {
     );
 }
 
-export default QnAListPage;
+export default QnAListPage;