[Python] FastAPI: pydantic validation error when returning model class

I'm using FastAPI, SQLModel and SQLAlchemy to build an API. I have the following model classes:

BaseModel

class CategoryBase(SQLModel):
name: str = Field(nullable=False, max_length=255)
position: int = Field(nullable=False, default=0)


DbModel

class Category(CategoryBase, table=True):
__tablename__ = "category"
id: int | None = Field(primary_key=True)


ReturnModel

class CategoryModel(BaseModel):
id: int
name: str
position: int



I want my API endpoint to return a list of CategoryModel with the following code:

@categories_router.get("", response_model=List[CategoryModel])
async def get_categories(*, session: Session = SessionDep):
categories = session.exec(
select(Category).order_by(Category.position)
).all()

return categories


However when I run the endpoint, I get the following error message:

fastapi.exceptions.ResponseValidationError: 2 validation errors:
{'type': 'missing', 'loc': ('response', 0, 'name'), 'msg': 'Field required', 'input': (Category(name='Einstieg', id=1, position=1),)}
{'type': 'missing', 'loc': ('response', 0, 'id'), 'msg': 'Field required', 'input': (Category(name='Einstieg', id=1, position=1),)}


During debugging I found out that the data is loaded correctly:

The id, name and position attributes are loaded correctly.

When removing the response_model attribute from the decorator, I get the follwing error message:

ValueError: [ValueError('dictionary update sequence element #0 has length 3; 2 is required'), TypeError('vars() argument must have __dict__ attribute')]


When using the deprecated query function, it does work as intended:

@categories_router.get("", response_model=List[CategoryModel])
async def get_categories(*, session: Session = SessionDep):
categories = session.query(Category).order_by(Category.position).all()
return categories


This is the correct response:

[
{
"id": 1,
"name": "Einstieg",
"position": 1
}
]


When using session.execute, it also works:

@categories_router.get("", response_model=List[CategoryModel])
async def get_categories(*, session: Session = SessionDep):
result = session.execute(
select(Category).order_by(Category.position)
).scalars().all()

return result


But session.execute is also deprecated. I don't want to use the deprecated API, but the documentation doesn't give me any indication of what is going wrong. What am I missing?

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