Sqlalchemy does not return an model object with `session.scalars` on mapped objects

Question

from attrs import define
from sqlalchemy.orm import (
    registry,
)
from sqlalchemy.sql import (
    schema,
    sqltypes,
)


@define(slots=False)
class Cat():
    id: int
    name: str


mapper_registry = registry()


cat_table = schema.Table(
    "cat",
    mapper_registry.metadata,
    schema.Column("id", sqltypes.Integer, primary_key=True),
    schema.Column("name", sqltypes.String, nullable=False),
)


mapper_registry.map_imperatively(models.Cat, cat_table)


async def main() -> None:
    ...
    model = Cat(1, "Meow")

    async with session_maker() as session:
        await session.add(model)  # Ok
        await session.commit()

        result = await session.scalar(cat_table.select())
        print(result)  # 1
        print(type(result))  # int

session.scalar returns int, not Cat

It looks like it worked with query:
https://github.com/cosmicpython/code/blob/master/src/allocation/adapters/repository.py#L48

I tried to get the object through scalars, but I need to construct it myself

Am I doing something wrong or is it supposed to be like this?

Asked By: LEv145

||

Source

Answer 1

I think you want:

result = (await session.scalar(select(Cat)))
# OR
result = (await session.scalars(select(Cat))).first()

Some other examples:

async def main() -> None:
    async_engine = create_async_engine(f"postgresql+asyncpg://{username}:{password}@/{db}", echo=False)
    async with async_engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)
    async_session = sessionmaker(async_engine, expire_on_commit=False, class_=AsyncSession)

    async with async_session() as session:
        cat_id = 1
        cat_name = "Meow"
        model = Cat(cat_id, cat_name)
        session.add(model)  # Ok
        await session.commit()

        def check(res, exp):
            assert res == exp
            print (res)

        result = await session.scalar(cat_table.select())
        check(result, 1)
        result = (await session.execute(cat_table.select())).first()
        check(result, (cat_id, cat_name))
        result = (await session.execute(select(cat_table))).first()
        check(result, (cat_id, cat_name))
        result = (await session.scalars(select(Cat))).first()
        check(result, model)
        result = (await session.scalar(select(Cat)))
        check(result, model)
        result = (await session.execute(select(Cat))).scalars().first()
        check(result, model)
        result = (await session.execute(select(Cat))).first()
        check(result, (model,))
        result = (await session.execute(select(Cat))).all()
        check(result, [(model,)])
        result = (await session.scalars(select(Cat))).all()
        check(result, [model])

Answered By: Ian Wilson

Sqlalchemy does not return an model object with `session.scalars` on mapped objects

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