examples.asyncio.async_orm_writeonly — SQLAlchemy 2.0 Documentation

Release: 2.0.49 | Release Date: April 3, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.asyncio.async_orm_writeonly

    """Illustrates using **write only relationships** for simpler handling
    of ORM collections under asyncio.

    """

    from __future__ import annotations

    import asyncio
    import datetime
    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 DeclarativeBase
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column
    from sqlalchemy.orm import relationship
    from sqlalchemy.orm import WriteOnlyMapped

    class Base(AsyncAttrs, DeclarativeBase):
        pass

    class A(Base):
        __tablename__ = "a"

        id: Mapped[int] = mapped_column(primary_key=True)
        data: Mapped[Optional[str]]
        create_date: Mapped[datetime.datetime] = mapped_column(
            server_default=func.now()
        )

        # collection relationships are declared with WriteOnlyMapped.  There
        # is no separate collection type
        bs: WriteOnlyMapped[B] = relationship()

    class B(Base):
        __tablename__ = "b"
        id: Mapped[int] = mapped_column(primary_key=True)
        a_id: Mapped[int] = mapped_column(ForeignKey("a.id"))
        data: Mapped[Optional[str]]

    async def async_main():
        """Main program function."""

        engine = create_async_engine(
            "postgresql+asyncpg://scott:tiger@localhost/test",
            echo=True,
        )

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

        async_session = async_sessionmaker(engine, expire_on_commit=False)

        async with async_session() as session:
            async with session.begin():
                # WriteOnlyMapped may be populated using any iterable,
                # e.g. lists, sets, etc.
                session.add_all(
                    [
                        A(bs=[B(), B()], data="a1"),
                        A(bs=[B()], data="a2"),
                        A(bs=[B(), B()], data="a3"),
                    ]
                )

            stmt = select(A)

            result = await session.scalars(stmt)

            for a1 in result:
                print(a1)
                print(f"created at: {a1.create_date}")

                # to iterate a collection, emit a SELECT statement
                for b1 in await session.scalars(a1.bs.select()):
                    print(b1)

            result = await session.stream(stmt)

            async for a1 in result.scalars():
                print(a1)

                # similar using "streaming" (server side cursors)
                async for b1 in (await session.stream(a1.bs.select())).scalars():
                    print(b1)

            await session.commit()
            result = await session.scalars(select(A).order_by(A.id))

            a1 = result.first()

            a1.data = "new data"

    asyncio.run(async_main())