Fancy solution for http://stackoverflow.com/questions/9084985/sqlalchemy-relationship-to-specific-columns/

column_rel_with_events.py

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, String, Integer, ForeignKey
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy import event
Base = declarative_base()
Session = sessionmaker()

class EmployeeType(Base):
    __tablename__ = 'employee_type'
    id = Column(Integer(), primary_key=True)
    name = Column(String(20))

class Employee(Base):
    __tablename__ = 'employee'
    id = Column(Integer(), primary_key=True)
    type_id = Column(Integer(), ForeignKey(EmployeeType.id))
    type = relationship(EmployeeType)

    _pending_type_name = None

    def _attach_pending_type(self):
        if self._pending_type_name is not None:
            self.type = session.query(EmployeeType).filter_by(name=self._pending_type_name).one()
            self._pending_type_name = None

    @property
    def type_name(self):
        if self._pending_type_name is not None:
            return self._pending_type_name
        if self.type is not None:
            return self.type.name
        return None

    @type_name.setter
    def type_name(self, value):
        if value is None:
            self.type = None
        else:
            self._pending_type_name = value
            session = Session.object_session(self)
            if session is not None:
                self._attach_pending_type()

@event.listens_for(Session, 'after_attach')
def _after_attach(session, instance):
    if isinstance(instance, Employee):
        instance._attach_pending_type()