main.py

# coding=utf-8

from sqlalchemy import create_engine, Column, Integer, DECIMAL, DATE, String, ForeignKey, Float, TEXT
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship, backref

db_schema = 'sqlite:///sqlite.db'
debug = True
engine = create_engine(db_schema, echo=debug)
create_session = sessionmaker(bind=engine)
Base = declarative_base()

class M(Base):
    __tablename__ = 'm'
    id = Column(Integer, primary_key=True)
    description_id = Column(Integer, ForeignKey('description.id'))
    description = relationship("Description", backref='ms')

class Description(Base):
    __tablename__ = 'description'
    id = Column(Integer, primary_key=True)
    name = Column(String(100))

def create_m(session, desc):
    return M(description_id=add_description(session, desc).id)

def add_description(session, description):
    return insert_into_db(session, Description(name=description))

def insert_into_db(session, ins):
    session.add(ins)
    session.flush()
    return ins

if __name__ == '__main__':
    Base.metadata.create_all(engine)
    session = create_session()
    m = create_m(session, 'desc')
    print m.description_id  # has value
    print m.description     # get None
    session.commit()
    print m.description_id  # has value
    print m.description     # get None