Listing or returning items with UoW raises sqlalchemy.orm.exc.DetachedInstanceError

[sevetseh28]

Hi. I have a simple list endpoint as follows:

@app.get("/items/", response_model=list[schemas.Item])
async def read_items(uow: AbstractUnitOfWork = Depends(get_uow)) -> List[models.Item]:
    with uow:
        items = uow.repo.list()
    return items

items is a list of models.Item which is my domain model (dataclass). But when exiting the uow (closing the session) the items' attributes are refreshed by the ORM automatically and I get the sqlalchemy.orm.exc.DetachedInstanceError exception.

I would expect such a dataclass not to mutate since it is not an ORM model.
What would be the best approach in this case?

The same applies for the creation of an item.

Thanks!

[sevetseh28]

Maybe using schemas.Item as a DTO like this, and convert:

def item_to_schema(item: models.Item) -> schemas.Item:
    return schemas.Item(id=item.id, title=item.title, description=item.description)

@app.get("/items/", response_model=list[schemas.Item])
async def read_items(uow: AbstractUnitOfWork = Depends(get_uow)) -> List[schemas.Item]:
    with uow:
        items = uow.repo.list()
        # Convert items to schemas.Item DTOs
        items_dtos = [item_to_schema(item) for item in items]
    return items_dtos

wdyt?

[instanceofmel]

A little late, but for future readers:

You have multiple options in this case, including the one @sevetseh28 provided. Other options:

• Return your list inside the with block
• Set expire_on_commit=False when creating the session. This will disable the reloading of attributes after commit. I think it's safe to disable it because a handler shouldn't reload anything after commiting anyways as the aggregate must be transactionally consistent and complete before committing.
• Use a view as described in Chapter 12