declarative_base.py

from sqlalchemy.orm import declarative_base, scoped_session, sessionmaker
from sqlalchemy.ext.declarative.base import _declarative_constructor
from sqlalchemy import Column, Integer, ForeignKey, create_engine


engine =  create_engine(# engine init code here)
session = scoped_session(sessionmaker(bind=engine))

class CustomBase(object):

    def __init__(self, **kwargs):
        _declarative_constructor(self, **kwargs)
        session.add(self)


Entity = declarative_base(cls=CustomBase, constructor=CustomBase.__init__)


class Item(Entity):

    id = Column(Integer, primary_key=True)
    type_id = Column(ForeignKey('item_type.id'), Integer)


class ItemType(Entity):

    id = Column(Integer, primary_key=True)