examples.association.basic_association — SQLAlchemy 2.0 Documentation

Release: 2.0.49 | Release Date: April 3, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.association.basic_association

    """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 __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.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"
        )

        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]

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

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

    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 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.scalars(
                select(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 = (
                select(Order)
                .join(OrderItem)
                .join(Item)
                .where(
                    Item.description == "MySQL Crowbar",
                    Item.price > OrderItem.price,
                )
            )

            print([order.customer_name for order in session.scalars(q)])