异步 SQLalchemy：访问急切加载的空关系会触发新的延迟加载，引发错误

Question

我正在使用 sqlalchemy + asyncpg，并且 'selectin' 预先加载。

我有与朋友有一对多关系的个人项目。

我将一个人插入到我的数据库中，没有相关的朋友条目。如果在同一个会话中我尝试从数据库中获取那个人，我可以很好地访问他们的静态（非关系）列，但无法访问 friends 关系。

我认为尝试访问 person.friends 会触发延迟加载，尽管它之前被强制执行为 selectin 加载。为什么是这样？我怎样才能避免它？

# Create the ORM model
class Person(Base):
    __tablename__ = 'items'
    id_ = Column(POSTGRES_UUID(as_uuid=True), primary_key=True)
    name = Column(String(32))
    friends = relationship('Friend', lazy='selectin')

# Create an instance
person_id = uuid4()
person = Person(id_=person_id, name='Alice') # Note that this Person's friends are not set

# Add to database
async with AsyncSession(engine, expire_on_commit=False) as session:
    try:
        session.begin()
        session.add(person)
        await session.commit()
    except:
        await session.rollback()
        raise
    # Get the added person from the database
    created_person = await session.get(person, person_id)
    print(created_person.id_) # Works fine
    print(created_person.friends) # Raises error

错误：

sqlalchemy.exc.MissingGreenlet: greenlet_spawn has not been called; can't call await_() here.
Was IO attempted in an unexpected place? (Background on this error at: https://sqlalche.me/e/14/xd2s)

Answer 1

解决方法是在get中使用populate_existing参数：

populate_existing – causes the method to unconditionally emit a SQL query and refresh the object with the newly loaded data, regardless of whether or not the object is already present.

替换

created_person = await session.get(person, person_id)

和

created_person = await session.get(person, person_id, populate_existing=True)

session.get documentation

另请参阅：https://github.com/sqlalchemy/sqlalchemy/issues/7176

异步 SQLalchemy：访问急切加载的空关系会触发新的延迟加载，引发错误

Async SQLalchemy: accessing eagerly-loaded empty relationship triggers new lazy-load, raising error

asynchronous

sqlalchemy

python-asyncio

asyncpg