examples.association.basic_association — SQLAlchemy 1.1 Documentation

Release: 1.1.18 | Release Date: March 6, 2018

# SQLAlchemy 1.1 Documentation

# Source code for examples.association.basic_association

    """basic_association.py

    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 (create_engine, Column, Integer, String, DateTime,
                            Float, ForeignKey, and_)
    from sqlalchemy.orm import 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])