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"""A basic example of using the association object pattern.
The association object pattern is a form of many-to-many which
associates additional data with each association between parent/child.
The example illustrates an "order", referencing a collection
of "items", with a particular price paid associated with each "item".
"""
from datetime import datetime
from sqlalchemy import (create_engine, MetaData, Table, Column, Integer,
String, DateTime, Float, ForeignKey, and_)
from sqlalchemy.orm import mapper, relationship, Session
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Order(Base):
__tablename__ = 'order'
order_id = Column(Integer, primary_key=True)
customer_name = Column(String(30), nullable=False)
order_date = Column(DateTime, nullable=False, default=datetime.now())
order_items = relationship("OrderItem", cascade="all, delete-orphan",
backref='order')
def __init__(self, customer_name):
self.customer_name = customer_name
class Item(Base):
__tablename__ = 'item'
item_id = Column(Integer, primary_key=True)
description = Column(String(30), nullable=False)
price = Column(Float, nullable=False)
def __init__(self, description, price):
self.description = description
self.price = price
def __repr__(self):
return 'Item(%r, %r)' % (
self.description, self.price
)
class OrderItem(Base):
__tablename__ = 'orderitem'
order_id = Column(Integer, ForeignKey('order.order_id'), primary_key=True)
item_id = Column(Integer, ForeignKey('item.item_id'), primary_key=True)
price = Column(Float, nullable=False)
def __init__(self, item, price=None):
self.item = item
self.price = price or item.price
item = relationship(Item, lazy='joined')
if __name__ == '__main__':
engine = create_engine('sqlite://')
Base.metadata.create_all(engine)
session = Session(engine)
# create catalog
tshirt, mug, hat, crowbar = (
Item('SA T-Shirt', 10.99),
Item('SA Mug', 6.50),
Item('SA Hat', 8.99),
Item('MySQL Crowbar', 16.99)
)
session.add_all([tshirt, mug, hat, crowbar])
session.commit()
# create an order
order = Order('john smith')
# add three OrderItem associations to the Order and save
order.order_items.append(OrderItem(mug))
order.order_items.append(OrderItem(crowbar, 10.99))
order.order_items.append(OrderItem(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])
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