[Python] Field validator not called on SQLModel

I have a FastAPI setup of the form:

class Foo(sqlmodel.SQLModel, table=True):
id: typing.Optional[int] = sqlmodel.Field(primary_key=True)
data: str

@pydantic.field_validator("data", mode="before")
def serialize_dict(cls, value):
if isinstance(value, dict):
return json.dumps(value)
return value


@app.post("/foos")
def create_foo(foo: Foo, session: sqlmodel.Session = fastapi.Depends(get_session)):
session.add(foo)
session.commit()
return fastapi.Response()


I then POST

{
"data": {
"bar": 5
}
}


to /foos. However, this is throwing a SQL exception because the data value couldn't be bound. After putting in some logging statements, I discovered that foo.data is a dict and not a str. In addition, I confirmed that my validator is never called.

Since SQLModel inherits from pydantic.BaseModel, I would have thought I could use such a validator. What am I missing?

This is sqlmodel 0.0.23 with pydantic 2.10.6.

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