# sqlalchemy.orm.exc.DetachedInstanceError: Parent instance <User> is not bound to a Session; lazy load operation of attribute 'posts' cannot proceed

- **ID:** `python/flask-sqlalchemy-detached-instance`
- **Domain:** python
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

在 Flask 会话关闭后访问了 ORM 对象的懒加载属性，导致 DetachedInstanceError。

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |

## Workarounds

1. **** (90% success)
   ```
   from sqlalchemy.orm import joinedload
user = db.session.query(User).options(joinedload(User.posts)).first()
   ```
2. **** (95% success)
   ```
   with app.app_context():
    user = db.session.get(User, 1)
    posts = user.posts  # 访问后再关闭会话
   ```

## Dead Ends

- **** — 如果关联数据很多，会降低性能，且可能遗漏。 (60% fail)
- **** — 重新附加可能引发其他冲突，且需要额外代码。 (70% fail)