fix: 修复任务服务SQLAlchemy异步加载错误

- create_task 和 get_tasks 现在使用 selectinload 预加载关联关系
- 避免懒加载导致的 MissingGreenlet 错误

backend/app/services/task_service.py

@@ -5,7 +5,7 @@ from typing import List, Optional
 from datetime import datetime
 from sqlalchemy import select, func, and_, case
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy.orm import joinedload
+from sqlalchemy.orm import joinedload, selectinload
 from app.models.task import Task, TaskCourse, TaskAssignment, TaskStatus, AssignmentStatus
 from app.models.course import Course
 from app.schemas.task import TaskCreate, TaskUpdate, TaskStatsResponse
@@ -44,7 +44,14 @@ class TaskService(BaseService[Task]):
             db.add(assignment)
 
         await db.commit()
-        await db.refresh(task)
+        
+        # 重新查询并加载关联关系（避免懒加载问题）
+        stmt = select(Task).where(Task.id == task.id).options(
+            selectinload(Task.course_links).selectinload(TaskCourse.course),
+            selectinload(Task.assignments)
+        )
+        result = await db.execute(stmt)
+        task = result.scalar_one()
         return task
 
     async def get_tasks(
@@ -61,6 +68,12 @@ class TaskService(BaseService[Task]):
             stmt = stmt.where(Task.status == status)
         
         stmt = stmt.order_by(Task.created_at.desc())
+        
+        # 加载关联关系
+        stmt = stmt.options(
+            selectinload(Task.course_links).selectinload(TaskCourse.course),
+            selectinload(Task.assignments)
+        )
 
         # 获取总数
         count_stmt = select(func.count()).select_from(Task).where(Task.is_deleted == False)
@@ -71,7 +84,7 @@ class TaskService(BaseService[Task]):
         # 分页
         stmt = stmt.offset((page - 1) * page_size).limit(page_size)
         result = await db.execute(stmt)
-        tasks = result.scalars().all()
+        tasks = result.unique().scalars().all()
 
         return tasks, total