examples.association.proxied_association — SQLAlchemy 2.0 Documentation

Release: 2.0.42 | Release Date: July 29, 2025

# SQLAlchemy 2.0 Documentation

# 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 __future__ import annotations

    from datetime import datetime

    from sqlalchemy import create_engine
    from sqlalchemy import ForeignKey
    from sqlalchemy import select
    from sqlalchemy import String
    from sqlalchemy.ext.associationproxy import association_proxy
    from sqlalchemy.ext.associationproxy import AssociationProxy
    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column
    from sqlalchemy.orm import relationship
    from sqlalchemy.orm import Session

    class Base(DeclarativeBase):
        pass

    class Order(Base):
        __tablename__ = "order"

        order_id: Mapped[int] = mapped_column(primary_key=True)
        customer_name: Mapped[str] = mapped_column(String(30))
        order_date: Mapped[datetime] = mapped_column(default=datetime.now())
        order_items: Mapped[list[OrderItem]] = relationship(
            cascade="all, delete-orphan", backref="order"
        )
        items: AssociationProxy[list[Item]] = association_proxy(
            "order_items", "item"
        )

        def __init__(self, customer_name: str) -> None:
            self.customer_name = customer_name

    class Item(Base):
        __tablename__ = "item"
        item_id: Mapped[int] = mapped_column(primary_key=True)
        description: Mapped[str] = mapped_column(String(30))
        price: Mapped[float]

        def __init__(self, description: str, price: float) -> None:
            self.description = description
            self.price = price

        def __repr__(self) -> str:
            return "Item({!r}, {!r})".format(self.description, self.price)

    class OrderItem(Base):
        __tablename__ = "orderitem"
        order_id: Mapped[int] = mapped_column(
            ForeignKey("order.order_id"), primary_key=True
        )
        item_id: Mapped[int] = mapped_column(
            ForeignKey("item.item_id"), primary_key=True
        )
        price: Mapped[float]

        item: Mapped[Item] = relationship(lazy="joined")

        def __init__(self, item: Item, price: float | None = None):
            self.item = item
            self.price = price or item.price

    if __name__ == "__main__":
        engine = create_engine("sqlite://")
        Base.metadata.create_all(engine)

        with Session(engine) as session:

            # 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.scalars(
                select(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_stmt = (
                select(Order)
                .join(OrderItem)
                .join(Item)
                .filter(Item.description == "MySQL Crowbar")
                .filter(Item.price > OrderItem.price)
            )
            print([o.customer_name for o in session.scalars(orders_stmt)])