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+# API Structure Guide - Curriculum Management
+
+## 📋 Tổng quan
+
+API được tổ chức theo cấu trúc phân cấp (hierarchy) và quan hệ nhiều-nhiều:
+
+### Cấu trúc phân cấp (1:N):
+```
+Category (Danh mục)
+  └── Subject (Môn học)
+       └── Chapter (Chương)
+            └── Lesson (Bài học)
+```
+
+### Quan hệ nhiều-nhiều (N:N):
+```
+Lesson (Bài học) ←→ Story (Câu chuyện)
+- Một lesson có thể có nhiều stories
+- Một story có thể được sử dụng trong nhiều lessons
+- Quản lý qua bảng pivot: lesson_stories
+```
+
+## 🎯 Nguyên tắc thiết kế
+
+### ✅ Điều NÊN làm:
+1. **Giữ nguyên cấu trúc phân tách**: Mỗi resource (category, subject, chapter, lesson) có router và controller riêng
+2. **Sử dụng nested routes**: Cho các thao tác liên quan đến parent-child relationship
+3. **RESTful naming**: Đặt tên routes theo chuẩn REST
+4. **Validation đầy đủ**: Kiểm tra parent resource tồn tại trước khi thao tác
+
+### ❌ Điều KHÔNG NÊN làm:
+1. **Không gộp chung tất cả vào 1 file**: Vi phạm Single Responsibility Principle
+2. **Không skip validation**: Luôn kiểm tra parent resource
+3. **Không quên clear cache**: Cache phải được xóa khi có thay đổi
+
+---
+
+## 📚 API Endpoints
+
+### 1. Categories API
+
+#### CRUD Operations
+```http
+GET    /api/categories              # Lấy danh sách categories
+POST   /api/categories              # Tạo category mới
+GET    /api/categories/:id          # Lấy chi tiết category
+PUT    /api/categories/:id          # Cập nhật category
+DELETE /api/categories/:id          # Xóa category
+```
+
+#### Nested Subject Operations
+```http
+GET    /api/categories/:categoryId/subjects                   # Lấy subjects trong category
+POST   /api/categories/:categoryId/subjects                   # Thêm subject vào category
+DELETE /api/categories/:categoryId/subjects/:subjectId        # Xóa subject khỏi category
+```
+
+**Ví dụ sử dụng:**
+```bash
+# Tạo subject mới trong category
+curl -X POST http://localhost:3000/api/categories/123e4567-e89b-12d3-a456-426614174000/subjects \
+  -H "Content-Type: application/json" \
+  -d '{
+    "subject_code": "ENG101",
+    "subject_name": "English Basics",
+    "description": "Basic English course"
+  }'
+
+# Xóa subject khỏi category
+curl -X DELETE http://localhost:3000/api/categories/123e4567-e89b-12d3-a456-426614174000/subjects/456e7890-e89b-12d3-a456-426614174001
+```
+
+---
+
+### 2. Subjects API
+
+#### CRUD Operations
+```http
+GET    /api/subjects                # Lấy danh sách subjects
+POST   /api/subjects                # Tạo subject mới
+GET    /api/subjects/:id            # Lấy chi tiết subject
+PUT    /api/subjects/:id            # Cập nhật subject
+DELETE /api/subjects/:id            # Xóa subject
+```
+
+#### Nested Chapter Operations
+```http
+GET    /api/subjects/:subjectId/chapters                    # Lấy chapters trong subject
+POST   /api/subjects/:subjectId/chapters                    # Thêm chapter vào subject
+DELETE /api/subjects/:subjectId/chapters/:chapterId         # Xóa chapter khỏi subject
+```
+
+**Ví dụ sử dụng:**
+```bash
+# Tạo chapter mới trong subject
+curl -X POST http://localhost:3000/api/subjects/456e7890-e89b-12d3-a456-426614174001/chapters \
+  -H "Content-Type: application/json" \
+  -d '{
+    "chapter_number": 1,
+    "chapter_title": "Introduction",
+    "chapter_description": "Getting started with English",
+    "display_order": 1
+  }'
+
+# Xóa chapter khỏi subject
+curl -X DELETE http://localhost:3000/api/subjects/456e7890-e89b-12d3-a456-426614174001/chapters/789e0123-e89b-12d3-a456-426614174002
+```
+
+---
+
+### 3. Chapters API
+
+#### CRUD Operations
+```http
+GET    /api/chapters                # Lấy danh sách chapters
+POST   /api/chapters                # Tạo chapter mới
+GET    /api/chapters/:id            # Lấy chi tiết chapter
+PUT    /api/chapters/:id            # Cập nhật chapter
+DELETE /api/chapters/:id            # Xóa chapter
+```
+
+#### Nested Lesson Operations
+```http
+GET    /api/chapters/:chapterId/lessons                     # Lấy lessons trong chapter
+POST   /api/chapters/:chapterId/lessons                     # Thêm lesson vào chapter
+DELETE /api/chapters/:chapterId/lessons/:lessonId           # Xóa lesson khỏi chapter
+```
+
+**Ví dụ sử dụng:**
+```bash
+# Tạo lesson mới trong chapter
+curl -X POST http://localhost:3000/api/chapters/789e0123-e89b-12d3-a456-426614174002/lessons \
+  -H "Content-Type: application/json" \
+  -d '{
+    "lesson_number": 1,
+    "lesson_title": "Alphabet A-F",
+    "lesson_type": "json_content",
+    "display_order": 1
+  }'
+
+# Xóa lesson khỏi chapter
+curl -X DELETE http://localhost:3000/api/chapters/789e0123-e89b-12d3-a456-426614174002/lessons/012e3456-e89b-12d3-a456-426614174003
+```
+
+---
+
+### 4. Lessons API
+
+#### CRUD Operations
+```http
+GET    /api/lessons                 # Lấy danh sách lessons
+POST   /api/lessons                 # Tạo lesson mới (cần chỉ định chapter_id)
+GET    /api/lessons/:id             # Lấy chi tiết lesson
+PUT    /api/lessons/:id             # Cập nhật lesson
+DELETE /api/lessons/:id             # Xóa lesson
+```
+
+#### Nested Story Operations (N:N Relationship)
+```http
+GET    /api/lessons/:lessonId/stories                     # Lấy stories trong lesson
+POST   /api/lessons/:lessonId/stories                     # Thêm story vào lesson
+PUT    /api/lessons/:lessonId/stories/:storyId            # Cập nhật story trong lesson
+DELETE /api/lessons/:lessonId/stories/:storyId            # Xóa story khỏi lesson
+```
+
+**Ví dụ sử dụng:**
+```bash
+# Thêm story vào lesson
+curl -X POST http://localhost:3000/api/lessons/012e3456-e89b-12d3-a456-426614174003/stories \
+  -H "Content-Type: application/json" \
+  -d '{
+    "story_id": "234e5678-e89b-12d3-a456-426614174004",
+    "display_order": 1,
+    "is_required": true
+  }'
+
+# Cập nhật story trong lesson
+curl -X PUT http://localhost:3000/api/lessons/012e3456-e89b-12d3-a456-426614174003/stories/234e5678-e89b-12d3-a456-426614174004 \
+  -H "Content-Type: application/json" \
+  -d '{
+    "display_order": 2,
+    "is_required": false
+  }'
+
+# Xóa story khỏi lesson
+curl -X DELETE http://localhost:3000/api/lessons/012e3456-e89b-12d3-a456-426614174003/stories/234e5678-e89b-12d3-a456-426614174004
+```
+
+---
+
+### 5. Stories API
+
+#### CRUD Operations
+```http
+GET    /api/stories                 # Lấy danh sách stories
+POST   /api/stories                 # Tạo story mới
+GET    /api/stories/:id             # Lấy chi tiết story
+PUT    /api/stories/:id             # Cập nhật story
+DELETE /api/stories/:id             # Xóa story
+```
+
+#### Nested Lesson Operations (N:N Relationship - Alternative Way)
+```http
+GET    /api/stories/:storyId/lessons                      # Lấy lessons sử dụng story này
+POST   /api/stories/:storyId/lessons                      # Thêm lesson vào story
+DELETE /api/stories/:storyId/lessons/:lessonId            # Xóa lesson khỏi story
+```
+
+**Ví dụ sử dụng:**
+```bash
+# Lấy danh sách lessons sử dụng story
+curl -X GET http://localhost:3000/api/stories/234e5678-e89b-12d3-a456-426614174004/lessons
+
+# Thêm lesson vào story (cách alternative)
+curl -X POST http://localhost:3000/api/stories/234e5678-e89b-12d3-a456-426614174004/lessons \
+  -H "Content-Type: application/json" \
+  -d '{
+    "lesson_id": "012e3456-e89b-12d3-a456-426614174003",
+    "display_order": 1,
+    "is_required": true
+  }'
+```
+
+---
+
+## 🔄 So sánh 2 cách tiếp cận
+
+### Cách 1: Nested Routes (✅ Được áp dụng)
+
+#### Quan hệ 1:N (Parent-Child)
+```http
+POST /api/categories/:categoryId/subjects
+POST /api/subjects/:subjectId/chapters
+POST /api/chapters/:chapterId/lessons
+```
+
+#### Quan hệ N:N (Many-to-Many)
+```http
+# Từ phía Lesson
+POST /api/lessons/:lessonId/stories
+GET  /api/lessons/:lessonId/stories
+PUT  /api/lessons/:lessonId/stories/:storyId
+DELETE /api/lessons/:lessonId/stories/:storyId
+
+# Từ phía Story (alternative)
+POST /api/stories/:storyId/lessons
+GET  /api/stories/:storyId/lessons
+DELETE /api/stories/:storyId/lessons/:lessonId
+```
+
+**Ưu điểm:**
+- ✅ Rõ ràng về mối quan hệ parent-child và many-to-many
+- ✅ Tự động validate parent tồn tại
+- ✅ RESTful và semantic
+- ✅ Dễ hiểu cho developers
+- ✅ Hỗ trợ quản lý pivot table metadata (display_order, is_required)
+
+**Nhược điểm:**
+- ⚠️ URL có thể dài hơn
+- ⚠️ Cần nhiều routes hơn
+
+### Cách 2: Flat Routes
+```http
+POST /api/subjects  (với category_id trong body)
+POST /api/chapters  (với subject_id trong body)
+POST /api/lessons   (với chapter_id trong body)
+```
+
+**Ưu điểm:**
+- ✅ URL ngắn hơn
+- ✅ Ít routes hơn
+
+**Nhược điểm:**
+- ❌ Không rõ ràng về mối quan hệ
+- ❌ Phải validate parent manually trong body
+- ❌ Kém RESTful
+
+---
+
+## 🎨 Patterns được áp dụng
+
+### 1. Controller Pattern
+Mỗi resource có controller riêng:
+```
+controllers/
+  ├── categoryController.js   # Quản lý Categories + nested Subjects
+  ├── subjectController.js    # Quản lý Subjects + nested Chapters
+  ├── chapterController.js    # Quản lý Chapters + nested Lessons
+  ├── lessonController.js     # Quản lý Lessons + nested Stories (N:N)
+  └── storyController.js      # Quản lý Stories + nested Lessons (N:N)
+```
+
+### 2. Route Organization
+Mỗi resource có route file riêng:
+```
+routes/
+  ├── categoryRoutes.js       # Category routes + nested Subject routes
+  ├── subjectRoutes.js        # Subject routes + nested Chapter routes
+  ├── chapterRoutes.js        # Chapter routes + nested Lesson routes
+  ├── lessonRoutes.js         # Lesson routes + nested Story routes
+  └── storyRoutes.js          # Story routes + nested Lesson routes
+```
+
+### 3. Model Relationships
+```javascript
+// 1:N Relationships
+Category.hasMany(Subject)
+Subject.hasMany(Chapter)
+Chapter.hasMany(Lesson)
+
+// N:N Relationship
+Lesson.belongsToMany(Story, { through: 'LessonStory' })
+Story.belongsToMany(Lesson, { through: 'LessonStory' })
+```
+
+### 4. Validation Pattern
+```javascript
+// Luôn validate parent trước khi thao tác child
+const parent = await ParentModel.findByPk(parentId);
+if (!parent) {
+  return res.status(404).json({
+    success: false,
+    message: 'Parent not found'
+  });
+}
+
+// Validate cả 2 phía trong quan hệ N:N
+const lesson = await Lesson.findByPk(lessonId);
+const story = await Story.findByPk(storyId);
+if (!lesson || !story) {
+  return res.status(404).json({
+    success: false,
+    message: 'Lesson or Story not found'
+  });
+}
+
+// Kiểm tra duplicate trong quan hệ N:N
+const existing = await LessonStory.findOne({
+  where: { lesson_id: lessonId, story_id: storyId }
+});
+if (existing) {
+  return res.status(400).json({
+    success: false,
+    message: 'Relationship already exists'
+  });
+}
+```
+
+### 5. Cache Invalidation Pattern
+```javascript
+// Xóa cache của cả parent và child (1:N)
+await cacheUtils.deletePattern(`parent:${parentId}:children:*`);
+await cacheUtils.deletePattern('children:list:*');
+
+// Xóa cache của cả 2 phía (N:N)
+await cacheUtils.deletePattern(`lesson:${lessonId}:stories:*`);
+await cacheUtils.deletePattern(`story:${storyId}:lessons:*`);
+```
+
+### 6. Pivot Table Pattern (N:N)
+```javascript
+// Bảng LessonStory với metadata
+{
+  lesson_id: UUID,
+  story_id: UUID,
+  display_order: INTEGER,    // Thứ tự hiển thị
+  is_required: BOOLEAN,      // Có bắt buộc không
+}
+
+// Truy vấn với pivot data
+const stories = await lesson.getStories({
+  joinTableAttributes: ['display_order', 'is_required']
+});
+```
+
+---
+
+## 🔍 Best Practices
+
+### 1. Đặt tên Routes
+```javascript
+// ✅ Đúng: RESTful naming
+router.post('/:categoryId/subjects', ...)
+router.delete('/:categoryId/subjects/:subjectId', ...)
+
+// ❌ Sai: Naming không chuẩn
+router.post('/:categoryId/addSubject', ...)
+router.delete('/:categoryId/removeSubject/:subjectId', ...)
+```
+
+### 2. Response Structure
+```javascript
+// ✅ Đúng: Consistent response structure
+{
+  "success": true,
+  "message": "Subject added to category successfully",
+  "data": { ...subject }
+}
+
+// ❌ Sai: Inconsistent
+{
+  "subject": { ...subject },
+  "msg": "OK"
+}
+```
+
+### 3. Error Handling
+```javascript
+// ✅ Đúng: Descriptive error messages
+if (!chapter) {
+  return res.status(404).json({
+    success: false,
+    message: 'Chapter not found in this subject'
+  });
+}
+
+// ❌ Sai: Generic error
+if (!chapter) {
+  return res.status(404).json({ error: 'Not found' });
+}
+```
+
+---
+
+## 📊 Request/Response Examples
+
+### Example 1: Tạo Subject trong Category
+
+**Request:**
+```http
+POST /api/categories/123e4567-e89b-12d3-a456-426614174000/subjects
+Content-Type: application/json
+
+{
+  "subject_code": "ENG101",
+  "subject_name": "English Basics",
+  "subject_name_en": "English Basics",
+  "description": "Basic English for beginners",
+  "is_active": true,
+  "is_premium": false
+}
+```
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "Subject added to category successfully",
+  "data": {
+    "id": "456e7890-e89b-12d3-a456-426614174001",
+    "category_id": "123e4567-e89b-12d3-a456-426614174000",
+    "subject_code": "ENG101",
+    "subject_name": "English Basics",
+    "subject_name_en": "English Basics",
+    "description": "Basic English for beginners",
+    "is_active": true,
+    "is_premium": false,
+    "created_at": "2026-02-26T10:00:00.000Z",
+    "updated_at": "2026-02-26T10:00:00.000Z"
+  }
+}
+```
+
+### Example 2: Xóa Chapter khỏi Subject
+
+**Request:**
+```http
+DELETE /api/subjects/456e7890-e89b-12d3-a456-426614174001/chapters/789e0123-e89b-12d3-a456-426614174002
+```
+
+**Response (Success):**
+```json
+{
+  "success": true,
+  "message": "Chapter removed from subject successfully"
+}
+```
+
+**Response (Error - Chapter có lessons):**
+```json
+{
+  "success": false,
+  "message": "Cannot delete chapter. It has 5 lesson(s). Delete lessons first."
+}
+```
+
+### Example 3: Thêm Story vào Lesson (N:N)
+
+**Request:**
+```http
+POST /api/lessons/012e3456-e89b-12d3-a456-426614174003/stories
+Content-Type: application/json
+
+{
+  "story_id": "234e5678-e89b-12d3-a456-426614174004",
+  "display_order": 1,
+  "is_required": true
+}
+```
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "Story đã được thêm vào bài học",
+  "data": {
+    "id": "345e6789-e89b-12d3-a456-426614174005",
+    "lesson_id": "012e3456-e89b-12d3-a456-426614174003",
+    "story_id": "234e5678-e89b-12d3-a456-426614174004",
+    "display_order": 1,
+    "is_required": true,
+    "created_at": "2026-02-26T11:00:00.000Z",
+    "updated_at": "2026-02-26T11:00:00.000Z"
+  }
+}
+```
+
+### Example 4: Lấy Stories trong Lesson
+
+**Request:**
+```http
+GET /api/lessons/012e3456-e89b-12d3-a456-426614174003/stories?page=1&limit=10
+```
+
+**Response:**
+```json
+{
+  "success": true,
+  "data": {
+    "lesson": {
+      "id": "012e3456-e89b-12d3-a456-426614174003",
+      "lesson_title": "Alphabet A-F",
+      "lesson_number": 1
+    },
+    "stories": [
+      {
+        "id": "234e5678-e89b-12d3-a456-426614174004",
+        "name": "The Greedy Cat",
+        "type": "story",
+        "thumbnail": "https://cdn.sena.tech/thumbs/greedy-cat.jpg",
+        "display_order": 1,
+        "is_required": true
+      }
+    ],
+    "pagination": {
+      "total": 1,
+      "page": 1,
+      "limit": 10,
+      "totalPages": 1
+    }
+  },
+  "cached": false
+}
+```
+
+### Example 5: Cập nhật Story trong Lesson
+
+**Request:**
+```http
+PUT /api/lessons/012e3456-e89b-12d3-a456-426614174003/stories/234e5678-e89b-12d3-a456-426614174004
+Content-Type: application/json
+
+{
+  "display_order": 2,
+  "is_required": false
+}
+```
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "Cập nhật thành công",
+  "data": {
+    "id": "345e6789-e89b-12d3-a456-426614174005",
+    "lesson_id": "012e3456-e89b-12d3-a456-426614174003",
+    "story_id": "234e5678-e89b-12d3-a456-426614174004",
+    "display_order": 2,
+    "is_required": false,
+    "updated_at": "2026-02-26T12:00:00.000Z"
+  }
+}
+```
+
+---
+
+## 🚀 Migration Guide
+
+Nếu bạn đang sử dụng cấu trúc cũ, đây là cách migrate:
+
+### Before (Cũ)
+```javascript
+// Tạo subject
+POST /api/subjects
+Body: { category_id: "xxx", subject_code: "ENG101", ... }
+
+// Tạo chapter
+POST /api/chapters
+Body: { subject_id: "yyy", chapter_number: 1, ... }
+```
+
+### After (Mới)
+```javascript
+// Cách 1: Sử dụng nested routes (khuyến nghị)
+POST /api/categories/:categoryId/subjects
+Body: { subject_code: "ENG101", ... }  // Không cần category_id
+
+POST /api/subjects/:subjectId/chapters
+Body: { chapter_number: 1, ... }  // Không cần subject_id
+
+// Cách 2: Vẫn dùng flat routes (vẫn hỗ trợ)
+POST /api/subjects
+Body: { category_id: "xxx", subject_code: "ENG101", ... }
+
+POST /api/chapters
+Body: { subject_id: "yyy", chapter_number: 1, ... }
+```
+
+---
+
+## 📝 Notes
+
+1. **Backward Compatibility**: Flat routes vẫn hoạt động bình thường
+2. **Nested routes**: Được khuyến nghị cho các thao tác mới
+3. **Cache Strategy**: Automatic cache invalidation được implement ở tất cả endpoints
+4. **Validation**: Parent resource luôn được validate trước khi thao tác
+
+---
+
+## 🔗 Related Documents
+
+- [SUBJECT_CHAPTER_LESSON_GUIDE.md](SUBJECT_CHAPTER_LESSON_GUIDE.md)
+- [LEARNING_CONTENT_GUIDE.md](LEARNING_CONTENT_GUIDE.md)
+- [SWAGGER_GUIDE.md](SWAGGER_GUIDE.md)
diff --git a/LESSON_STORY_GUIDE.md b/LESSON_STORY_GUIDE.md
new file mode 100644
index 0000000..406bafb
--- /dev/null
+++ b/LESSON_STORY_GUIDE.md
@@ -0,0 +1,307 @@
+# Lesson-Story N:N Relationship Guide
+
+## 📋 Tổng quan
+
+Quan hệ **Many-to-Many (N:N)** giữa `Lesson` và `Story`:
+- Một lesson có thể chứa nhiều stories
+- Một story có thể được sử dụng trong nhiều lessons
+- Quản lý qua bảng pivot: `lesson_stories`
+
+## 🗂️ Database Schema
+
+### Bảng: lesson_stories
+```sql
+CREATE TABLE lesson_stories (
+  id              CHAR(36) PRIMARY KEY,
+  lesson_id       CHAR(36) NOT NULL,
+  story_id        CHAR(36) NOT NULL,
+  display_order   INT DEFAULT 0,
+  is_required     BOOLEAN DEFAULT TRUE,
+  created_at      TIMESTAMP,
+  updated_at      TIMESTAMP,
+  UNIQUE(lesson_id, story_id)
+);
+```
+
+### Metadata trong Pivot Table
+- `display_order`: Thứ tự hiển thị story trong lesson
+- `is_required`: Story có bắt buộc phải hoàn thành không
+
+## 🔌 API Endpoints
+
+### 1. Quản lý từ phía Lesson
+
+#### Lấy danh sách stories trong lesson
+```http
+GET /api/lessons/:lessonId/stories?page=1&limit=20
+```
+
+**Response:**
+```json
+{
+  "success": true,
+  "data": {
+    "lesson": {
+      "id": "xxx",
+      "lesson_title": "Alphabet A-F",
+      "lesson_number": 1
+    },
+    "stories": [
+      {
+        "id": "yyy",
+        "name": "The Greedy Cat",
+        "type": "story",
+        "display_order": 1,
+        "is_required": true
+      }
+    ],
+    "pagination": { ... }
+  }
+}
+```
+
+#### Thêm story vào lesson
+```http
+POST /api/lessons/:lessonId/stories
+Content-Type: application/json
+
+{
+  "story_id": "story-uuid",
+  "display_order": 1,
+  "is_required": true
+}
+```
+
+#### Cập nhật story trong lesson
+```http
+PUT /api/lessons/:lessonId/stories/:storyId
+Content-Type: application/json
+
+{
+  "display_order": 2,
+  "is_required": false
+}
+```
+
+#### Xóa story khỏi lesson
+```http
+DELETE /api/lessons/:lessonId/stories/:storyId
+```
+
+---
+
+### 2. Quản lý từ phía Story (Alternative)
+
+#### Lấy danh sách lessons sử dụng story
+```http
+GET /api/stories/:storyId/lessons?page=1&limit=20
+```
+
+**Response:**
+```json
+{
+  "success": true,
+  "data": {
+    "story": {
+      "id": "yyy",
+      "name": "The Greedy Cat",
+      "type": "story"
+    },
+    "lessons": [
+      {
+        "id": "xxx",
+        "lesson_title": "Alphabet A-F",
+        "display_order": 1,
+        "is_required": true
+      }
+    ],
+    "pagination": { ... }
+  }
+}
+```
+
+#### Thêm lesson vào story
+```http
+POST /api/stories/:storyId/lessons
+Content-Type: application/json
+
+{
+  "lesson_id": "lesson-uuid",
+  "display_order": 1,
+  "is_required": true
+}
+```
+
+#### Xóa lesson khỏi story
+```http
+DELETE /api/stories/:storyId/lessons/:lessonId
+```
+
+## 💡 Use Cases
+
+### Use Case 1: Tạo lesson với nhiều stories
+```bash
+# 1. Tạo lesson
+POST /api/chapters/:chapterId/lessons
+{
+  "lesson_number": 1,
+  "lesson_title": "Learning ABC",
+  "lesson_type": "json_content"
+}
+
+# 2. Thêm story thứ nhất
+POST /api/lessons/{lesson_id}/stories
+{
+  "story_id": "story-1-id",
+  "display_order": 1,
+  "is_required": true
+}
+
+# 3. Thêm story thứ hai
+POST /api/lessons/{lesson_id}/stories
+{
+  "story_id": "story-2-id",
+  "display_order": 2,
+  "is_required": false
+}
+```
+
+### Use Case 2: Tái sử dụng story cho nhiều lessons
+```bash
+# Story "The Greedy Cat" được sử dụng trong 3 lessons khác nhau
+
+# Lesson 1 - Grade 1
+POST /api/lessons/lesson-1-id/stories
+{
+  "story_id": "greedy-cat-id",
+  "display_order": 1,
+  "is_required": true
+}
+
+# Lesson 2 - Grade 2
+POST /api/lessons/lesson-2-id/stories
+{
+  "story_id": "greedy-cat-id",
+  "display_order": 2,
+  "is_required": false
+}
+
+# Lesson 3 - Review lesson
+POST /api/lessons/lesson-3-id/stories
+{
+  "story_id": "greedy-cat-id",
+  "display_order": 1,
+  "is_required": true
+}
+```
+
+### Use Case 3: Cập nhật thứ tự stories trong lesson
+```bash
+# Đổi thứ tự từ 1 sang 3
+PUT /api/lessons/{lesson_id}/stories/{story_id}
+{
+  "display_order": 3
+}
+
+# Đánh dấu không bắt buộc
+PUT /api/lessons/{lesson_id}/stories/{story_id}
+{
+  "is_required": false
+}
+```
+
+## 🎯 Best Practices
+
+### 1. Naming Convention
+- `display_order` bắt đầu từ 1, tăng dần
+- Để lại khoảng trống giữa các order (1, 5, 10...) để dễ insert sau
+
+### 2. Validation
+```javascript
+// Check duplicate trước khi thêm
+const existing = await LessonStory.findOne({
+  where: { lesson_id, story_id }
+});
+
+if (existing) {
+  return res.status(400).json({
+    message: 'Story đã tồn tại trong lesson này'
+  });
+}
+```
+
+### 3. Cache Strategy
+```javascript
+// Clear cache của cả 2 phía khi có thay đổi
+await cacheUtils.deletePattern(`lesson:${lessonId}:stories:*`);
+await cacheUtils.deletePattern(`story:${storyId}:lessons:*`);
+```
+
+### 4. Query Optimization
+```javascript
+// Sử dụng include để eager load
+const lessons = await Lesson.findAll({
+  include: [{
+    model: Story,
+    as: 'stories',
+    through: {
+      attributes: ['display_order', 'is_required']
+    }
+  }]
+});
+```
+
+## ⚠️ Important Notes
+
+### Cascade Delete
+- Khi xóa lesson → Tự động xóa relationships trong `lesson_stories`
+- Khi xóa story → Tự động xóa relationships trong `lesson_stories`
+- **KHÔNG** xóa story/lesson khi xóa relationship
+
+### Transaction Safety
+```javascript
+const transaction = await sequelize.transaction();
+try {
+  // Add multiple stories to lesson
+  await LessonStory.bulkCreate([
+    { lesson_id, story_id: 'story1', display_order: 1 },
+    { lesson_id, story_id: 'story2', display_order: 2 },
+  ], { transaction });
+  
+  await transaction.commit();
+} catch (error) {
+  await transaction.rollback();
+  throw error;
+}
+```
+
+### Performance Tips
+1. **Index**: lesson_id và story_id đều có index
+2. **Pagination**: Luôn dùng limit khi query
+3. **Cache**: Cache kết quả 30 phút (1800s)
+4. **Eager Loading**: Load stories cùng lesson khi cần
+
+## 🔍 Troubleshooting
+
+### Issue 1: Duplicate Error
+```
+Error: Unique constraint violation
+```
+**Solution**: Story đã tồn tại trong lesson, kiểm tra trước khi thêm
+
+### Issue 2: Foreign Key Error
+```
+Error: Cannot add or update a child row
+```
+**Solution**: lesson_id hoặc story_id không tồn tại, validate trước
+
+### Issue 3: Order Conflict
+```
+Warning: Multiple stories have same display_order
+```
+**Solution**: Cập nhật display_order để unique trong lesson
+
+## 📚 Related Documents
+- [API_STRUCTURE_GUIDE.md](../API_STRUCTURE_GUIDE.md)
+- [STORY_GUIDE.md](../STORY_GUIDE.md)
+- [SUBJECT_CHAPTER_LESSON_GUIDE.md](../SUBJECT_CHAPTER_LESSON_GUIDE.md)
diff --git a/controllers/categoryController.js b/controllers/categoryController.js
index 408ae68..ae651ea 100644
--- a/controllers/categoryController.js
+++ b/controllers/categoryController.js
@@ -311,7 +311,7 @@ class CategoryController {
       }
 
       // Build query conditions
-      const where = { id: id };
+      const where = { category_id: id };
       if (is_active !== undefined) where.is_active = is_active === 'true';
 
       // Query subjects
@@ -349,6 +349,90 @@ class CategoryController {
       next(error);
     }
   }
+
+  /**
+   * Add subject to category (Create subject within category context)
+   */
+  async addSubjectToCategory(req, res, next) {
+    try {
+      const { categoryId } = req.params;
+      const subjectData = req.body;
+
+      // Check if category exists
+      const category = await Categories.findByPk(categoryId);
+      if (!category) {
+        return res.status(404).json({
+          success: false,
+          message: 'Category not found',
+        });
+      }
+
+      // Create subject with category_id
+      const subject = await Subject.create({
+        ...subjectData,
+        category_id: categoryId,
+      });
+
+      // Clear cache
+      await cacheUtils.deletePattern('subjects:list:*');
+      await cacheUtils.deletePattern(`category:${categoryId}:subjects:*`);
+
+      res.status(201).json({
+        success: true,
+        message: 'Subject added to category successfully',
+        data: subject,
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
+
+  /**
+   * Remove subject from category (Delete subject within category context)
+   */
+  async removeSubjectFromCategory(req, res, next) {
+    try {
+      const { categoryId, subjectId } = req.params;
+
+      // Check if category exists
+      const category = await Categories.findByPk(categoryId);
+      if (!category) {
+        return res.status(404).json({
+          success: false,
+          message: 'Category not found',
+        });
+      }
+
+      // Find subject
+      const subject = await Subject.findOne({
+        where: { 
+          id: subjectId,
+          category_id: categoryId,
+        },
+      });
+
+      if (!subject) {
+        return res.status(404).json({
+          success: false,
+          message: 'Subject not found in this category',
+        });
+      }
+
+      await subject.destroy();
+
+      // Clear cache
+      await cacheUtils.delete(`subject:${subjectId}`);
+      await cacheUtils.deletePattern('subjects:list:*');
+      await cacheUtils.deletePattern(`category:${categoryId}:subjects:*`);
+
+      res.json({
+        success: true,
+        message: 'Subject removed from category successfully',
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
 }
 
 module.exports = new CategoryController();
diff --git a/controllers/chapterController.js b/controllers/chapterController.js
index 4e5a5e4..b8f54c0 100644
--- a/controllers/chapterController.js
+++ b/controllers/chapterController.js
@@ -330,6 +330,98 @@ class ChapterController {
       next(error);
     }
   }
+
+  /**
+   * Add lesson to chapter (Create lesson within chapter context)
+   */
+  async addLessonToChapter(req, res, next) {
+    try {
+      const { chapterId } = req.params;
+      const lessonData = req.body;
+
+      // Check if chapter exists
+      const chapter = await Chapter.findByPk(chapterId);
+      if (!chapter) {
+        return res.status(404).json({
+          success: false,
+          message: 'Chapter not found',
+        });
+      }
+
+      // Validate required fields
+      if (!lessonData.lesson_number || !lessonData.lesson_title) {
+        return res.status(400).json({
+          success: false,
+          message: 'lesson_number and lesson_title are required',
+        });
+      }
+
+      // Create lesson with chapter_id
+      const lesson = await Lesson.create({
+        ...lessonData,
+        chapter_id: chapterId,
+      });
+
+      // Clear cache
+      await cacheUtils.deletePattern('lessons:*');
+      await cacheUtils.deletePattern(`chapter:${chapterId}:lessons:*`);
+
+      res.status(201).json({
+        success: true,
+        message: 'Lesson added to chapter successfully',
+        data: lesson,
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
+
+  /**
+   * Remove lesson from chapter (Delete lesson within chapter context)
+   */
+  async removeLessonFromChapter(req, res, next) {
+    try {
+      const { chapterId, lessonId } = req.params;
+
+      // Check if chapter exists
+      const chapter = await Chapter.findByPk(chapterId);
+      if (!chapter) {
+        return res.status(404).json({
+          success: false,
+          message: 'Chapter not found',
+        });
+      }
+
+      // Find lesson
+      const lesson = await Lesson.findOne({
+        where: { 
+          id: lessonId,
+          chapter_id: chapterId,
+        },
+      });
+
+      if (!lesson) {
+        return res.status(404).json({
+          success: false,
+          message: 'Lesson not found in this chapter',
+        });
+      }
+
+      await lesson.destroy();
+
+      // Clear cache
+      await cacheUtils.deletePattern('lessons:*');
+      await cacheUtils.deletePattern(`lesson:${lessonId}*`);
+      await cacheUtils.deletePattern(`chapter:${chapterId}:lessons:*`);
+
+      res.json({
+        success: true,
+        message: 'Lesson removed from chapter successfully',
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
 }
 
 module.exports = new ChapterController();
diff --git a/controllers/lessonController.js b/controllers/lessonController.js
index 444d92f..33d89a2 100644
--- a/controllers/lessonController.js
+++ b/controllers/lessonController.js
@@ -1,5 +1,7 @@
 const { Lesson, Chapter, Subject, Game } = require('../models');
 const { Op } = require('sequelize');
+const { Story, LessonStory } = require('../models');
+const { cacheUtils } = require('../config/redis');
 
 /**
  * Lesson Controller
@@ -384,6 +386,243 @@ class LessonController {
       next(error);
     }
   }
+
+  /**
+   * Lấy danh sách stories trong một lesson
+   */
+  async getStoriesByLesson(req, res, next) {
+    try {
+      const { lessonId } = req.params;
+      const { page = 1, limit = 20 } = req.query;
+      const offset = (page - 1) * limit;
+
+      const cacheKey = `lesson:${lessonId}:stories:${page}:${limit}`;
+      
+      const cached = await cacheUtils.get(cacheKey);
+      if (cached) {
+        return res.json({
+          success: true,
+          data: cached,
+          cached: true,
+        });
+      }
+
+      // Check if lesson exists
+      const lesson = await Lesson.findByPk(lessonId);
+      if (!lesson) {
+        return res.status(404).json({
+          success: false,
+          message: 'Không tìm thấy bài học',
+        });
+      }
+
+      // Get stories with pivot data
+      const { count, rows } = await Story.findAndCountAll({
+        include: [
+          {
+            model: Lesson,
+            as: 'lessons',
+            where: { id: lessonId },
+            through: {
+              attributes: ['display_order', 'is_required'],
+            },
+            attributes: [],
+          },
+        ],
+        limit: parseInt(limit),
+        offset: parseInt(offset),
+        order: [[{ model: Lesson, as: 'lessons' }, LessonStory, 'display_order', 'ASC']],
+      });
+
+      const result = {
+        lesson: {
+          id: lesson.id,
+          lesson_title: lesson.lesson_title,
+          lesson_number: lesson.lesson_number,
+        },
+        stories: rows.map(story => ({
+          ...story.toJSON(),
+          display_order: story.lessons[0]?.LessonStory?.display_order,
+          is_required: story.lessons[0]?.LessonStory?.is_required,
+        })),
+        pagination: {
+          total: count,
+          page: parseInt(page),
+          limit: parseInt(limit),
+          totalPages: Math.ceil(count / limit),
+        },
+      };
+
+      await cacheUtils.set(cacheKey, result, 1800);
+
+      res.json({
+        success: true,
+        data: result,
+        cached: false,
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
+
+  /**
+   * Thêm story vào lesson
+   */
+  async addStoryToLesson(req, res, next) {
+    try {
+      const { lessonId } = req.params;
+      const { story_id, display_order = 0, is_required = true } = req.body;
+
+      // Validate required fields
+      if (!story_id) {
+        return res.status(400).json({
+          success: false,
+          message: 'story_id is required',
+        });
+      }
+
+      // Check if lesson exists
+      const lesson = await Lesson.findByPk(lessonId);
+      if (!lesson) {
+        return res.status(404).json({
+          success: false,
+          message: 'Không tìm thấy bài học',
+        });
+      }
+
+      // Check if story exists
+      const story = await Story.findByPk(story_id);
+      if (!story) {
+        return res.status(404).json({
+          success: false,
+          message: 'Không tìm thấy story',
+        });
+      }
+
+      // Check if relationship already exists
+      const existing = await LessonStory.findOne({
+        where: {
+          lesson_id: lessonId,
+          story_id: story_id,
+        },
+      });
+
+      if (existing) {
+        return res.status(400).json({
+          success: false,
+          message: 'Story đã tồn tại trong bài học này',
+        });
+      }
+
+      // Create relationship
+      const lessonStory = await LessonStory.create({
+        lesson_id: lessonId,
+        story_id: story_id,
+        display_order,
+        is_required,
+      });
+
+      // Clear cache
+      await cacheUtils.deletePattern(`lesson:${lessonId}:stories:*`);
+      await cacheUtils.deletePattern(`story:${story_id}:lessons:*`);
+
+      res.status(201).json({
+        success: true,
+        message: 'Story đã được thêm vào bài học',
+        data: lessonStory,
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
+
+  /**
+   * Xóa story khỏi lesson
+   */
+  async removeStoryFromLesson(req, res, next) {
+    try {
+      const { lessonId, storyId } = req.params;
+
+      // Check if lesson exists
+      const lesson = await Lesson.findByPk(lessonId);
+      if (!lesson) {
+        return res.status(404).json({
+          success: false,
+          message: 'Không tìm thấy bài học',
+        });
+      }
+
+      // Find relationship
+      const lessonStory = await LessonStory.findOne({
+        where: {
+          lesson_id: lessonId,
+          story_id: storyId,
+        },
+      });
+
+      if (!lessonStory) {
+        return res.status(404).json({
+          success: false,
+          message: 'Story không tồn tại trong bài học này',
+        });
+      }
+
+      await lessonStory.destroy();
+
+      // Clear cache
+      await cacheUtils.deletePattern(`lesson:${lessonId}:stories:*`);
+      await cacheUtils.deletePattern(`story:${storyId}:lessons:*`);
+
+      res.json({
+        success: true,
+        message: 'Story đã được xóa khỏi bài học',
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
+
+  /**
+   * Cập nhật thông tin story trong lesson (display_order, is_required)
+   */
+  async updateStoryInLesson(req, res, next) {
+    try {
+      const { lessonId, storyId } = req.params;
+      const { display_order, is_required } = req.body;
+
+      // Find relationship
+      const lessonStory = await LessonStory.findOne({
+        where: {
+          lesson_id: lessonId,
+          story_id: storyId,
+        },
+      });
+
+      if (!lessonStory) {
+        return res.status(404).json({
+          success: false,
+          message: 'Story không tồn tại trong bài học này',
+        });
+      }
+
+      // Update
+      await lessonStory.update({
+        ...(display_order !== undefined && { display_order }),
+        ...(is_required !== undefined && { is_required }),
+      });
+
+      // Clear cache
+      await cacheUtils.deletePattern(`lesson:${lessonId}:stories:*`);
+
+      res.json({
+        success: true,
+        message: 'Cập nhật thành công',
+        data: lessonStory,
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
 }
 
 module.exports = new LessonController();
diff --git a/controllers/storyController.js b/controllers/storyController.js
index 1536ec6..848a027 100644
--- a/controllers/storyController.js
+++ b/controllers/storyController.js
@@ -1,5 +1,7 @@
 const { Story, sequelize } = require('../models');
 const { Op } = require('sequelize');
+const { Lesson, LessonStory } = require('../models');
+const { cacheUtils } = require('../config/redis');
 
 /**
  * CREATE: Add new story
@@ -733,3 +735,213 @@ exports.getStoryGuide = async (req, res) => {
     });
   }
 };
+
+/**
+ * Lấy danh sách lessons sử dụng story này
+ */
+exports.getLessonsByStory = async (req, res) => {
+  try {
+    const { storyId } = req.params;
+    const { page = 1, limit = 20 } = req.query;
+    const offset = (parseInt(page) - 1) * parseInt(limit);
+
+    const cacheKey = `story:${storyId}:lessons:${page}:${limit}`;
+    
+    const cached = await cacheUtils.get(cacheKey);
+    if (cached) {
+      return res.json({
+        success: true,
+        data: cached,
+        cached: true,
+      });
+    }
+
+    // Check if story exists
+    const story = await Story.findByPk(storyId);
+    if (!story) {
+      return res.status(404).json({
+        success: false,
+        message: 'Story not found',
+      });
+    }
+
+    // Get lessons with pivot data
+    const { count, rows } = await Lesson.findAndCountAll({
+      include: [
+        {
+          model: Story,
+          as: 'stories',
+          where: { id: storyId },
+          through: {
+            attributes: ['display_order', 'is_required'],
+          },
+          attributes: [],
+        },
+      ],
+      limit: parseInt(limit),
+      offset: parseInt(offset),
+      order: [[{ model: Story, as: 'stories' }, LessonStory, 'display_order', 'ASC']],
+    });
+
+    const result = {
+      story: {
+        id: story.id,
+        name: story.name,
+        type: story.type,
+      },
+      lessons: rows.map(lesson => ({
+        ...lesson.toJSON(),
+        display_order: lesson.stories[0]?.LessonStory?.display_order,
+        is_required: lesson.stories[0]?.LessonStory?.is_required,
+      })),
+      pagination: {
+        total: count,
+        page: parseInt(page),
+        limit: parseInt(limit),
+        totalPages: Math.ceil(count / parseInt(limit)),
+      },
+    };
+
+    await cacheUtils.set(cacheKey, result, 1800);
+
+    res.json({
+      success: true,
+      data: result,
+      cached: false,
+    });
+  } catch (error) {
+    console.error('Error fetching lessons by story:', error);
+    res.status(500).json({
+      success: false,
+      message: 'Failed to fetch lessons',
+      error: error.message,
+    });
+  }
+};
+
+/**
+ * Thêm lesson vào story (alternative way)
+ */
+exports.addLessonToStory = async (req, res) => {
+  try {
+    const { storyId } = req.params;
+    const { lesson_id, display_order = 0, is_required = true } = req.body;
+
+    // Validate required fields
+    if (!lesson_id) {
+      return res.status(400).json({
+        success: false,
+        message: 'lesson_id is required',
+      });
+    }
+
+    // Check if story exists
+    const story = await Story.findByPk(storyId);
+    if (!story) {
+      return res.status(404).json({
+        success: false,
+        message: 'Story not found',
+      });
+    }
+
+    // Check if lesson exists
+    const lesson = await Lesson.findByPk(lesson_id);
+    if (!lesson) {
+      return res.status(404).json({
+        success: false,
+        message: 'Lesson not found',
+      });
+    }
+
+    // Check if relationship already exists
+    const existing = await LessonStory.findOne({
+      where: {
+        lesson_id: lesson_id,
+        story_id: storyId,
+      },
+    });
+
+    if (existing) {
+      return res.status(400).json({
+        success: false,
+        message: 'Lesson đã sử dụng story này',
+      });
+    }
+
+    // Create relationship
+    const lessonStory = await LessonStory.create({
+      lesson_id: lesson_id,
+      story_id: storyId,
+      display_order,
+      is_required,
+    });
+
+    // Clear cache
+    await cacheUtils.deletePattern(`story:${storyId}:lessons:*`);
+    await cacheUtils.deletePattern(`lesson:${lesson_id}:stories:*`);
+
+    res.status(201).json({
+      success: true,
+      message: 'Lesson đã được thêm vào story',
+      data: lessonStory,
+    });
+  } catch (error) {
+    console.error('Error adding lesson to story:', error);
+    res.status(500).json({
+      success: false,
+      message: 'Failed to add lesson to story',
+      error: error.message,
+    });
+  }
+};
+
+/**
+ * Xóa lesson khỏi story
+ */
+exports.removeLessonFromStory = async (req, res) => {
+  try {
+    const { storyId, lessonId } = req.params;
+
+    // Check if story exists
+    const story = await Story.findByPk(storyId);
+    if (!story) {
+      return res.status(404).json({
+        success: false,
+        message: 'Story not found',
+      });
+    }
+
+    // Find relationship
+    const lessonStory = await LessonStory.findOne({
+      where: {
+        lesson_id: lessonId,
+        story_id: storyId,
+      },
+    });
+
+    if (!lessonStory) {
+      return res.status(404).json({
+        success: false,
+        message: 'Lesson không sử dụng story này',
+      });
+    }
+
+    await lessonStory.destroy();
+
+    // Clear cache
+    await cacheUtils.deletePattern(`story:${storyId}:lessons:*`);
+    await cacheUtils.deletePattern(`lesson:${lessonId}:stories:*`);
+
+    res.json({
+      success: true,
+      message: 'Lesson đã được xóa khỏi story',
+    });
+  } catch (error) {
+    console.error('Error removing lesson from story:', error);
+    res.status(500).json({
+      success: false,
+      message: 'Failed to remove lesson from story',
+      error: error.message,
+    });
+  }
+};
diff --git a/controllers/subjectController.js b/controllers/subjectController.js
index 2bb0441..01d596f 100644
--- a/controllers/subjectController.js
+++ b/controllers/subjectController.js
@@ -349,6 +349,108 @@ class SubjectController {
       next(error);
     }
   }
+
+  /**
+   * Add chapter to subject (Create chapter within subject context)
+   */
+  async addChapterToSubject(req, res, next) {
+    try {
+      const { subjectId } = req.params;
+      const chapterData = req.body;
+
+      // Check if subject exists
+      const subject = await Subject.findByPk(subjectId);
+      if (!subject) {
+        return res.status(404).json({
+          success: false,
+          message: 'Subject not found',
+        });
+      }
+
+      // Validate required fields
+      if (!chapterData.chapter_number || !chapterData.chapter_title) {
+        return res.status(400).json({
+          success: false,
+          message: 'chapter_number and chapter_title are required',
+        });
+      }
+
+      // Create chapter with subject_id
+      const chapter = await Chapter.create({
+        ...chapterData,
+        subject_id: subjectId,
+      });
+
+      // Clear cache
+      await cacheUtils.deletePattern('chapters:*');
+      await cacheUtils.deletePattern(`subject:${subjectId}:chapters:*`);
+
+      res.status(201).json({
+        success: true,
+        message: 'Chapter added to subject successfully',
+        data: chapter,
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
+
+  /**
+   * Remove chapter from subject (Delete chapter within subject context)
+   */
+  async removeChapterFromSubject(req, res, next) {
+    try {
+      const { subjectId, chapterId } = req.params;
+
+      // Check if subject exists
+      const subject = await Subject.findByPk(subjectId);
+      if (!subject) {
+        return res.status(404).json({
+          success: false,
+          message: 'Subject not found',
+        });
+      }
+
+      // Find chapter
+      const chapter = await Chapter.findOne({
+        where: { 
+          id: chapterId,
+          subject_id: subjectId,
+        },
+      });
+
+      if (!chapter) {
+        return res.status(404).json({
+          success: false,
+          message: 'Chapter not found in this subject',
+        });
+      }
+
+      // Check if chapter has lessons
+      const Lesson = require('../models').Lesson;
+      const lessonsCount = await Lesson.count({ where: { chapter_id: chapterId } });
+      if (lessonsCount > 0) {
+        return res.status(400).json({
+          success: false,
+          message: `Cannot delete chapter. It has ${lessonsCount} lesson(s). Delete lessons first.`,
+        });
+      }
+
+      await chapter.destroy();
+
+      // Clear cache
+      await cacheUtils.deletePattern('chapters:*');
+      await cacheUtils.deletePattern(`chapter:${chapterId}*`);
+      await cacheUtils.deletePattern(`subject:${subjectId}:chapters:*`);
+
+      res.json({
+        success: true,
+        message: 'Chapter removed from subject successfully',
+      });
+    } catch (error) {
+      next(error);
+    }
+  }
 }
 
 module.exports = new SubjectController();
diff --git a/fix-bcrypt-complete.bat b/fix-bcrypt-complete.bat
deleted file mode 100644
index 5a00d2b..0000000
--- a/fix-bcrypt-complete.bat
+++ /dev/null
@@ -1,17 +0,0 @@
-@echo off
-REM Complete bcrypt fix for Windows
-REM This will remove node_modules and reinstall everything
-
-echo ================================================================
-echo   Complete Bcrypt Fix Script
-echo   Server: root@senaai.tech
-echo ================================================================
-echo.
-
-ssh root@senaai.tech "cd /var/www/services/sena_db_api && echo 'Removing node_modules...' && rm -rf node_modules package-lock.json pnpm-lock.yaml yarn.lock && echo 'Installing with npm...' && npm install && echo 'Rebuilding bcrypt...' && npm rebuild bcrypt --build-from-source && echo 'Restarting PM2...' && pm2 restart all && pm2 list"
-
-echo.
-echo ================================================================
-echo   Done! Check the output above.
-echo ================================================================
-pause
diff --git a/migrations/20260226_create_lesson_stories.sql b/migrations/20260226_create_lesson_stories.sql
new file mode 100644
index 0000000..99ff684
--- /dev/null
+++ b/migrations/20260226_create_lesson_stories.sql
@@ -0,0 +1,35 @@
+-- Migration: Create lesson_stories pivot table
+-- Description: N-N relationship between Lessons and Stories
+-- Date: 2026-02-26
+
+CREATE TABLE IF NOT EXISTS `lesson_stories` (
+  `id` CHAR(36) NOT NULL PRIMARY KEY,
+  `lesson_id` CHAR(36) NOT NULL,
+  `story_id` CHAR(36) NOT NULL,
+  `display_order` INT DEFAULT 0 COMMENT 'Thứ tự hiển thị story trong lesson',
+  `is_required` BOOLEAN DEFAULT TRUE COMMENT 'Story này có bắt buộc hoàn thành không',
+  `created_at` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  `updated_at` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  
+  -- Indexes
+  INDEX `idx_lesson_id` (`lesson_id`),
+  INDEX `idx_story_id` (`story_id`),
+  INDEX `idx_display_order` (`display_order`),
+  
+  -- Unique constraint to prevent duplicate relationships
+  UNIQUE KEY `unique_lesson_story` (`lesson_id`, `story_id`),
+  
+  -- Foreign keys
+  CONSTRAINT `fk_lesson_stories_lesson`
+    FOREIGN KEY (`lesson_id`)
+    REFERENCES `lessons` (`id`)
+    ON DELETE CASCADE
+    ON UPDATE CASCADE,
+    
+  CONSTRAINT `fk_lesson_stories_story`
+    FOREIGN KEY (`story_id`)
+    REFERENCES `stories` (`id`)
+    ON DELETE CASCADE
+    ON UPDATE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
+COMMENT='Pivot table for N-N relationship between Lessons and Stories';
diff --git a/models/LessonStory.js b/models/LessonStory.js
new file mode 100644
index 0000000..e2b81f6
--- /dev/null
+++ b/models/LessonStory.js
@@ -0,0 +1,56 @@
+const { DataTypes } = require('sequelize');
+const { sequelize } = require('../config/database');
+
+/**
+ * LessonStory Model - Pivot table for N-N relationship
+ * Many-to-Many: Lessons <-> Stories
+ */
+const LessonStory = sequelize.define('lesson_stories', {
+  id: {
+    type: DataTypes.UUID,
+    defaultValue: DataTypes.UUIDV4,
+    primaryKey: true,
+  },
+  lesson_id: {
+    type: DataTypes.UUID,
+    allowNull: false,
+    comment: 'ID của bài học',
+  },
+  story_id: {
+    type: DataTypes.UUID,
+    allowNull: false,
+    comment: 'ID của story',
+  },
+  display_order: {
+    type: DataTypes.INTEGER,
+    defaultValue: 0,
+    comment: 'Thứ tự hiển thị story trong lesson',
+  },
+  is_required: {
+    type: DataTypes.BOOLEAN,
+    defaultValue: true,
+    comment: 'Story này có bắt buộc hoàn thành không',
+  },
+  created_at: {
+    type: DataTypes.DATE,
+    allowNull: false,
+    defaultValue: DataTypes.NOW,
+  },
+  updated_at: {
+    type: DataTypes.DATE,
+    allowNull: false,
+    defaultValue: DataTypes.NOW,
+  },
+}, {
+  tableName: 'lesson_stories',
+  timestamps: true,
+  underscored: true,
+  indexes: [
+    { fields: ['lesson_id'] },
+    { fields: ['story_id'] },
+    { fields: ['lesson_id', 'story_id'], unique: true },
+    { fields: ['display_order'] },
+  ],
+});
+
+module.exports = LessonStory;
diff --git a/models/index.js b/models/index.js
index 834a2da..80d336c 100644
--- a/models/index.js
+++ b/models/index.js
@@ -33,6 +33,7 @@ const Game = require('./Game');
 const GameType = require('./GameType');
 const LessonComponentProgress = require('./LessonComponentProgress');
 const LessonLeaderboard = require('./LessonLeaderboard');
+const LessonStory = require('./LessonStory');
 
 // Group 3.2: Vocabulary System (NEW)
 const Vocab = require('./Vocab');
@@ -162,6 +163,24 @@ const setupRelationships = () => {
   Chapter.hasMany(Lesson, { foreignKey: 'chapter_id', as: 'lessons' });
   Lesson.belongsTo(Chapter, { foreignKey: 'chapter_id', as: 'chapter' });
   
+  // Lesson <-> Story (N:N through LessonStory)
+  Lesson.belongsToMany(Story, {
+    through: LessonStory,
+    foreignKey: 'lesson_id',
+    otherKey: 'story_id',
+    as: 'stories'
+  });
+  Story.belongsToMany(Lesson, {
+    through: LessonStory,
+    foreignKey: 'story_id',
+    otherKey: 'lesson_id',
+    as: 'lessons'
+  });
+  
+  // Direct access to pivot table
+  LessonStory.belongsTo(Lesson, { foreignKey: 'lesson_id', as: 'lesson' });
+  LessonStory.belongsTo(Story, { foreignKey: 'story_id', as: 'story' });
+  
   // Lesson Progress relationships
   LessonComponentProgress.belongsTo(UsersAuth, { foreignKey: 'user_id', as: 'user' });
   LessonComponentProgress.belongsTo(Lesson, { foreignKey: 'lesson_id', as: 'lesson' });
@@ -295,6 +314,7 @@ module.exports = {
   GameType,
   LessonComponentProgress,
   LessonLeaderboard,
+  LessonStory,
   
   // Group 3.2: Vocabulary System (NEW)
   Vocab,
diff --git a/routes/categoryRoutes.js b/routes/categoryRoutes.js
index f034220..60df480 100644
--- a/routes/categoryRoutes.js
+++ b/routes/categoryRoutes.js
@@ -33,4 +33,11 @@ router.put('/:id', categoryController.updateCategory);
 // DELETE /api/categories/:id - Delete category
 router.delete('/:id', categoryController.deleteCategory);
 
+// ============ Nested Subject Routes ============
+// POST /api/categories/:categoryId/subjects - Add subject to category
+router.post('/:categoryId/subjects', categoryController.addSubjectToCategory);
+
+// DELETE /api/categories/:categoryId/subjects/:subjectId - Remove subject from category
+router.delete('/:categoryId/subjects/:subjectId', categoryController.removeSubjectFromCategory);
+
 module.exports = router;
diff --git a/routes/chapterRoutes.js b/routes/chapterRoutes.js
index 665547b..d3c6c4c 100644
--- a/routes/chapterRoutes.js
+++ b/routes/chapterRoutes.js
@@ -25,4 +25,11 @@ router.put('/:id', chapterController.updateChapter);
 // Delete chapter
 router.delete('/:id', chapterController.deleteChapter);
 
+// ============ Nested Lesson Routes ============
+// POST /api/chapters/:chapterId/lessons - Add lesson to chapter
+router.post('/:chapterId/lessons', chapterController.addLessonToChapter);
+
+// DELETE /api/chapters/:chapterId/lessons/:lessonId - Remove lesson from chapter
+router.delete('/:chapterId/lessons/:lessonId', chapterController.removeLessonFromChapter);
+
 module.exports = router;
diff --git a/routes/lessonRoutes.js b/routes/lessonRoutes.js
index 8c3aac1..36f62d4 100644
--- a/routes/lessonRoutes.js
+++ b/routes/lessonRoutes.js
@@ -247,4 +247,137 @@ router.delete('/:id', lessonController.deleteLesson);
  */
 router.post('/:id/complete', lessonController.completeLesson);
 
+// ============ Nested Story Routes ============
+/**
+ * @swagger
+ * /api/lessons/{lessonId}/stories:
+ *   get:
+ *     tags: [Lessons]
+ *     summary: Lấy danh sách stories trong một lesson
+ *     parameters:
+ *       - in: path
+ *         name: lessonId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *       - in: query
+ *         name: page
+ *         schema:
+ *           type: integer
+ *           default: 1
+ *       - in: query
+ *         name: limit
+ *         schema:
+ *           type: integer
+ *           default: 20
+ *     responses:
+ *       200:
+ *         description: Danh sách stories trong lesson
+ */
+router.get('/:lessonId/stories', lessonController.getStoriesByLesson);
+
+/**
+ * @swagger
+ * /api/lessons/{lessonId}/stories:
+ *   post:
+ *     tags: [Lessons]
+ *     summary: Thêm story vào lesson
+ *     security:
+ *       - bearerAuth: []
+ *     parameters:
+ *       - in: path
+ *         name: lessonId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *     requestBody:
+ *       required: true
+ *       content:
+ *         application/json:
+ *           schema:
+ *             type: object
+ *             required:
+ *               - story_id
+ *             properties:
+ *               story_id:
+ *                 type: string
+ *                 format: uuid
+ *               display_order:
+ *                 type: integer
+ *                 default: 0
+ *               is_required:
+ *                 type: boolean
+ *                 default: true
+ *     responses:
+ *       201:
+ *         description: Story đã được thêm vào lesson
+ */
+router.post('/:lessonId/stories', lessonController.addStoryToLesson);
+
+/**
+ * @swagger
+ * /api/lessons/{lessonId}/stories/{storyId}:
+ *   put:
+ *     tags: [Lessons]
+ *     summary: Cập nhật story trong lesson (display_order, is_required)
+ *     security:
+ *       - bearerAuth: []
+ *     parameters:
+ *       - in: path
+ *         name: lessonId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *       - in: path
+ *         name: storyId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *     requestBody:
+ *       content:
+ *         application/json:
+ *           schema:
+ *             type: object
+ *             properties:
+ *               display_order:
+ *                 type: integer
+ *               is_required:
+ *                 type: boolean
+ *     responses:
+ *       200:
+ *         description: Cập nhật thành công
+ */
+router.put('/:lessonId/stories/:storyId', lessonController.updateStoryInLesson);
+
+/**
+ * @swagger
+ * /api/lessons/{lessonId}/stories/{storyId}:
+ *   delete:
+ *     tags: [Lessons]
+ *     summary: Xóa story khỏi lesson
+ *     security:
+ *       - bearerAuth: []
+ *     parameters:
+ *       - in: path
+ *         name: lessonId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *       - in: path
+ *         name: storyId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *     responses:
+ *       200:
+ *         description: Story đã được xóa khỏi lesson
+ */
+router.delete('/:lessonId/stories/:storyId', lessonController.removeStoryFromLesson);
+
 module.exports = router;
diff --git a/routes/storyRoutes.js b/routes/storyRoutes.js
index a65c537..640ad3c 100644
--- a/routes/storyRoutes.js
+++ b/routes/storyRoutes.js
@@ -386,4 +386,100 @@ router.put('/:id', storyController.updateStory);
  */
 router.delete('/:id', storyController.deleteStory);
 
+// ============ Nested Lesson Routes ============
+/**
+ * @swagger
+ * /api/stories/{storyId}/lessons:
+ *   get:
+ *     tags: [Stories]
+ *     summary: Lấy danh sách lessons sử dụng story này
+ *     parameters:
+ *       - in: path
+ *         name: storyId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *       - in: query
+ *         name: page
+ *         schema:
+ *           type: integer
+ *           default: 1
+ *       - in: query
+ *         name: limit
+ *         schema:
+ *           type: integer
+ *           default: 20
+ *     responses:
+ *       200:
+ *         description: Danh sách lessons sử dụng story
+ */
+router.get('/:storyId/lessons', storyController.getLessonsByStory);
+
+/**
+ * @swagger
+ * /api/stories/{storyId}/lessons:
+ *   post:
+ *     tags: [Stories]
+ *     summary: Thêm lesson vào story (alternative way)
+ *     security:
+ *       - bearerAuth: []
+ *     parameters:
+ *       - in: path
+ *         name: storyId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *     requestBody:
+ *       required: true
+ *       content:
+ *         application/json:
+ *           schema:
+ *             type: object
+ *             required:
+ *               - lesson_id
+ *             properties:
+ *               lesson_id:
+ *                 type: string
+ *                 format: uuid
+ *               display_order:
+ *                 type: integer
+ *                 default: 0
+ *               is_required:
+ *                 type: boolean
+ *                 default: true
+ *     responses:
+ *       201:
+ *         description: Lesson đã được thêm vào story
+ */
+router.post('/:storyId/lessons', storyController.addLessonToStory);
+
+/**
+ * @swagger
+ * /api/stories/{storyId}/lessons/{lessonId}:
+ *   delete:
+ *     tags: [Stories]
+ *     summary: Xóa lesson khỏi story
+ *     security:
+ *       - bearerAuth: []
+ *     parameters:
+ *       - in: path
+ *         name: storyId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *       - in: path
+ *         name: lessonId
+ *         required: true
+ *         schema:
+ *           type: string
+ *           format: uuid
+ *     responses:
+ *       200:
+ *         description: Lesson đã được xóa khỏi story
+ */
+router.delete('/:storyId/lessons/:lessonId', storyController.removeLessonFromStory);
+
 module.exports = router;
diff --git a/routes/subjectRoutes.js b/routes/subjectRoutes.js
index 0f27a23..4ea4ce6 100644
--- a/routes/subjectRoutes.js
+++ b/routes/subjectRoutes.js
@@ -33,4 +33,11 @@ router.put('/:id', subjectController.updateSubject);
 // DELETE /api/subjects/:id - Delete subject
 router.delete('/:id', subjectController.deleteSubject);
 
+// ============ Nested Chapter Routes ============
+// POST /api/subjects/:subjectId/chapters - Add chapter to subject
+router.post('/:subjectId/chapters', subjectController.addChapterToSubject);
+
+// DELETE /api/subjects/:subjectId/chapters/:chapterId - Remove chapter from subject
+router.delete('/:subjectId/chapters/:chapterId', subjectController.removeChapterFromSubject);
+
 module.exports = router;