Release: 1.4.0b1 pre release | Release Date: unreleased

SQLAlchemy 1.4 Documentation

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Source code for examples.association.proxied_association

"""Same example as basic_association, adding in
usage of :mod:`sqlalchemy.ext.associationproxy` to make explicit references
to ``OrderItem`` optional.


"""

from datetime import datetime

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.associationproxy import association_proxy
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"
    )
    items = association_proxy("order_items", "item")

    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 items via the association proxy.
    # the OrderItem is created automatically.
    order.items.append(mug)
    order.items.append(hat)

    # add an OrderItem explicitly.
    order.order_items.append(OrderItem(crowbar, 10.99))

    session.add(order)
    session.commit()

    # query the order, print items
    order = session.query(Order).filter_by(customer_name="john smith").one()

    # print items based on the OrderItem collection directly
    print(
        [
            (assoc.item.description, assoc.price, assoc.item.price)
            for assoc in order.order_items
        ]
    )

    # print items based on the "proxied" items collection
    print([(item.description, item.price) for item in order.items])

    # print customers who bought 'MySQL Crowbar' on sale
    orders = (
        session.query(Order)
        .join("order_items", "item")
        .filter(Item.description == "MySQL Crowbar")
        .filter(Item.price > OrderItem.price)
    )
    print([o.customer_name for o in orders])