gistfile1.py

import datetime

from pony.orm import *

db = Database('sqlite', 'data.sqlite', create_db=True)

class Base(db.Entity):
	created = Optional(datetime.datetime, default=datetime.datetime.now)


class A(Base):
	x = Optional(int)
	y = Optional(int)


class B(Base):
	y = Optional(int)
	z = Optional(int)
	

db.generate_mapping(create_tables=True)