add AsyncAttrs

Added a new helper mixin :class:`_asyncio.AsyncAttrs` that seeks to improve
the use of lazy-loader and other expired or deferred ORM attributes with
asyncio, providing a simple attribute accessor that provides an ``await``
interface to any ORM attribute, whether or not it needs to emit SQL.

Change-Id: I1427b288dc28319c854372643066c491b9ee8dc0
References: #9731

1 files changed, 11 insertions, 4 deletions

diff --git a/examples/asyncio/async_orm.py b/examples/asyncio/async_orm.py
index 66501e545..eabc0250d 100644
--- a/examples/asyncio/async_orm.py
+++ b/examples/asyncio/async_orm.py
@@ -12,15 +12,18 @@ from typing import Optional
 from sqlalchemy import ForeignKey
 from sqlalchemy import func
 from sqlalchemy.ext.asyncio import async_sessionmaker
+from sqlalchemy.ext.asyncio import AsyncAttrs
 from sqlalchemy.ext.asyncio import create_async_engine
 from sqlalchemy.future import select
-from sqlalchemy.orm import declarative_base
+from sqlalchemy.orm import DeclarativeBase
 from sqlalchemy.orm import Mapped
 from sqlalchemy.orm import mapped_column
 from sqlalchemy.orm import relationship
 from sqlalchemy.orm import selectinload
 
-Base = declarative_base()
+
+class Base(AsyncAttrs, DeclarativeBase):
+    pass
 
 
 class A(Base):
@@ -31,7 +34,7 @@ class A(Base):
     create_date: Mapped[datetime.datetime] = mapped_column(
         server_default=func.now()
     )
-    bs: Mapped[List[B]] = relationship(lazy="raise")
+    bs: Mapped[List[B]] = relationship()
 
 
 class B(Base):
@@ -93,11 +96,15 @@ async def async_main():
 
         result = await session.scalars(select(A).order_by(A.id))
 
-        a1 = result.first()
+        a1 = result.one()
 
         a1.data = "new data"
 
         await session.commit()
 
+        # use the AsyncAttrs interface to accommodate for a lazy load
+        for b1 in await a1.awaitable_attrs.bs:
+            print(b1)
+
 
 asyncio.run(async_main())