"""A basic example of using the association object pattern. The association object pattern is a richer form of a many-to-many relationship. The model will be an ecommerce example. We will have an Order, which represents a set of Items purchased by a user. Each Item has a price. However, the Order must store its own price for each Item, representing the price paid by the user for that particular order, which is independent of the price on each Item (since those can change). """ from datetime import datetime from sqlalchemy import (create_engine, MetaData, Table, Column, Integer, String, DateTime, Numeric, ForeignKey, and_) from sqlalchemy.orm import mapper, relationship, Session # Uncomment these to watch database activity. #import logging #logging.basicConfig(format='%(message)s') #logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO) engine = create_engine('sqlite:///') metadata = MetaData(engine) orders = Table('orders', metadata, Column('order_id', Integer, primary_key=True), Column('customer_name', String(30), nullable=False), Column('order_date', DateTime, nullable=False, default=datetime.now()), ) items = Table('items', metadata, Column('item_id', Integer, primary_key=True), Column('description', String(30), nullable=False), Column('price', Numeric(8, 2), nullable=False) ) orderitems = Table('orderitems', metadata, Column('order_id', Integer, ForeignKey('orders.order_id'), primary_key=True), Column('item_id', Integer, ForeignKey('items.item_id'), primary_key=True), Column('price', Numeric(8, 2), nullable=False) ) metadata.create_all() class Order(object): def __init__(self, customer_name): self.customer_name = customer_name class Item(object): def __init__(self, description, price): self.description = description self.price = price def __repr__(self): return 'Item(%s, %s)' % (repr(self.description), repr(self.price)) class OrderItem(object): def __init__(self, item, price=None): self.item = item self.price = price or item.price mapper(Order, orders, properties={ 'order_items': relationship(OrderItem, cascade="all, delete-orphan", backref='order') }) mapper(Item, items) mapper(OrderItem, orderitems, properties={ 'item': relationship(Item, lazy='joined') }) session = Session() # create our catalog session.add(Item('SA T-Shirt', 10.99)) session.add(Item('SA Mug', 6.50)) session.add(Item('SA Hat', 8.99)) session.add(Item('MySQL Crowbar', 16.99)) session.commit() # function to return items from the DB def item(name): return session.query(Item).filter_by(description=name).one() # create an order order = Order('john smith') # add three OrderItem associations to the Order and save order.order_items.append(OrderItem(item('SA Mug'))) order.order_items.append(OrderItem(item('MySQL Crowbar'), 10.99)) order.order_items.append(OrderItem(item('SA Hat'))) session.add(order) session.commit() # query the order, print items order = session.query(Order).filter_by(customer_name='john smith').one() print [(order_item.item.description, order_item.price) for order_item in order.order_items] # print customers who bought 'MySQL Crowbar' on sale q = session.query(Order).join('order_items', 'item') q = q.filter(and_(Item.description == 'MySQL Crowbar', Item.price > OrderItem.price)) print [order.customer_name for order in q]