"""Illustrate a many-to-many relationship between an
"Order" and a collection of "Item" objects, associating a purchase price
with each via an association object called "OrderItem"
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 and_
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import DateTime
from sqlalchemy import Float
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session
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])