basic_association.py

SQLAlchemy. 经典的Python ORM框架。学习必看。

"""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).
"""

import logging
from datetime import datetime

from sqlalchemy import *
from sqlalchemy.orm import *

# Uncomment these to watch database activity.
#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': relation(OrderItem, cascade="all, delete-orphan",
                            backref='order')
})
mapper(Item, items)
mapper(OrderItem, orderitems, properties={
    'item': relation(Item, lazy=False)
})

session = create_session()

# create our catalog
session.save(Item('SA T-Shirt', 10.99))
session.save(Item('SA Mug', 6.50))
session.save(Item('SA Hat', 8.99))
session.save(Item('MySQL Crowbar', 16.99))
session.flush()

# 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.save(order)
session.flush()

session.clear()

# 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]