如何访问与异步 sqlalchemy 的关系？

Question

import asyncio

from sqlalchemy import Column
from sqlalchemy import DateTime
from sqlalchemy import ForeignKey
from sqlalchemy import func
from sqlalchemy import Integer
from sqlalchemy import String
from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy.ext.asyncio import create_async_engine
from sqlalchemy.future import select
from sqlalchemy.orm import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.orm import selectinload
from sqlalchemy.orm import sessionmaker

engine = create_async_engine(
        "postgresql+asyncpg://user:pass@localhost/db",
        echo=True,
    )


# expire_on_commit=False will prevent attributes from being expired
# after commit.
async_session = sessionmaker(
    engine, expire_on_commit=False, class_=AsyncSession
)


Base = declarative_base()

class A(Base):
    __tablename__ = "a"

    id = Column(Integer, primary_key=True)
    name = Column(String, unique=True)
    data = Column(String)
    create_date = Column(DateTime, server_default=func.now())
    bs = relationship("B")

    # required in order to access columns with server defaults
    # or SQL expression defaults, subsequent to a flush, without
    # triggering an expired load
    __mapper_args__ = {"eager_defaults": True}


class B(Base):
    __tablename__ = "b"
    id = Column(Integer, primary_key=True)
    a_id = Column(ForeignKey("a.id"))
    data = Column(String)
    
    
       

async with engine.begin() as conn:
    await conn.run_sync(Base.metadata.drop_all)
    await conn.run_sync(Base.metadata.create_all)


async with async_session() as session:
    async with session.begin():
        session.add_all(
            [
                A(bs=[B(), B()], data="a1"),
                A(bs=[B()], data="a2"),
            ]
        )


async with async_session() as session:
    result = await session.execute(select(A).order_by(A.id))
    a1 = result.scalars().first()

    # no issue: 
    print(a1.name, a1.data)

    # throws error:
    print(a1.bs)

尝试访问 a1.bs 出现此错误：

     59     current = greenlet.getcurrent()
     60     if not isinstance(current, _AsyncIoGreenlet):
---> 61         raise exc.MissingGreenlet(
     62             "greenlet_spawn has not been called; can't call await_() here. "
     63             "Was IO attempted in an unexpected place?"

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

是这样的：

from sqlalchemy.orm import selectinload

async with async_session() as session:
    result = await session.execute(select(A).order_by(A.id)
                                            .options(selectinload(A.bs)))
    a = result.scalars().first()

    print(a.bs)

密钥正在使用 selectinload method to prevent implicit IO

更新

selectinload 有一些替代方法，例如 joinedload、lazyload。我仍在努力理解差异。

如何访问与异步 sqlalchemy 的关系？

how to access relationships with async sqlalchemy?

python

async-await

sqlalchemy

python-asyncio

asyncpg