Sqlalchemy sqlalchemy.orm.exc.UnmappedColumnError

Base Abstract Class: ConsultantModel.py

class ConsultantModel(DeclarativeBase):
__tablename__         = 'consultant'
__abstract__          = True

Sub class 1: Consultant.py

class Consultant(ConsultantModel):

Sub class 2: Staff.py

class Staff(ConsultantModel):

TG Controller: SetupController.py

`

from phb.model import DBSession
class SetupController(BaseController):

    def _before(self, *args, **kw):
       self.user_id    = request.identity['user'].user_id
       self.consultant = DBSession.query(Consultant).filter_by(user_id=self.user_id).one()

    def staff(self):
       staff = Staff()
       staff.name = 'john'
       DBSession.add(staff)
       self.__updateStep()


    def __updateStep(self):
       self.consultant.completed = self.current_step + 1
       DBSession.add(self.consultant)

`

The problematic code is self.consultant.completed = self.current_step + 1 No errors occur if I remove this line.