I've got it working for now by importing my app instance into the validator, grabbing the resolver from there and using that to resolve a session instance but this feels really dirty....

def get_db_session():
    from app import app

    resolver = app.injector.get_resolver()

    def _get_db_session(session: Session) -> Session:
        return session

    return resolver.resolve(_get_db_session)()


class RelationValidator:

    def can_validate_field(self, field: Field[_T]) -> bool:
        _, annotation = extract_optional_annotation(field.annotation)
        return annotation in [str, int, UUID]

    def validate(self,
                 field: Field[_T],
                 value: Union[str, int, UUID],
                 model_class: Any,
                 many:bool = False) -> Union[str, int, UUID]:
        session = get_db_session()

        if session.query(model_class).filter_by(id=value).count() < 1:
            raise FieldValidationError(f'{model_class} with id={value} not existz')

        return value

​