examples.generic_associations.table_per_association — SQLAlchemy 2.0 Documentation

Release: 2.0.48 | Release Date: March 2, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.generic_associations.table_per_association

    """Illustrates a mixin which provides a generic association
    via a individually generated association tables for each parent class.
    The associated objects themselves are persisted in a single table
    shared among all parents.

    This configuration has the advantage that all Address
    rows are in one table, so that the definition of "Address"
    can be maintained in one place.   The association table
    contains the foreign key to Address so that Address
    has no dependency on the system.

    """

    from sqlalchemy import Column
    from sqlalchemy import create_engine
    from sqlalchemy import ForeignKey
    from sqlalchemy import Table
    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import declared_attr
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column
    from sqlalchemy.orm import relationship
    from sqlalchemy.orm import Session

    class Base(DeclarativeBase):
        """Base class which provides automated table name
        and surrogate primary key column.
        """

        @declared_attr
        def __tablename__(cls):
            return cls.__name__.lower()

        id: Mapped[int] = mapped_column(primary_key=True)

    class Address(Base):
        """The Address class.

        This represents all address records in a
        single table.
        """

        street: Mapped[str]
        city: Mapped[str]
        zip: Mapped[str]

        def __repr__(self):
            return "%s(street=%r, city=%r, zip=%r)" % (
                self.__class__.__name__,
                self.street,
                self.city,
                self.zip,
            )

    class HasAddresses:
        """HasAddresses mixin, creates a new address_association
        table for each parent.

        """

        @declared_attr
        def addresses(cls):
            address_association = Table(
                "%s_addresses" % cls.__tablename__,
                cls.metadata,
                Column("address_id", ForeignKey("address.id"), primary_key=True),
                Column(
                    "%s_id" % cls.__tablename__,
                    ForeignKey("%s.id" % cls.__tablename__),
                    primary_key=True,
                ),
            )
            return relationship(Address, secondary=address_association)

    class Customer(HasAddresses, Base):
        name: Mapped[str]

    class Supplier(HasAddresses, Base):
        company_name: Mapped[str]

    engine = create_engine("sqlite://", echo=True)
    Base.metadata.create_all(engine)

    session = Session(engine)

    session.add_all(
        [
            Customer(
                name="customer 1",
                addresses=[
                    Address(
                        street="123 anywhere street", city="New York", zip="10110"
                    ),
                    Address(
                        street="40 main street", city="San Francisco", zip="95732"
                    ),
                ],
            ),
            Supplier(
                company_name="Ace Hammers",
                addresses=[
                    Address(street="2569 west elm", city="Detroit", zip="56785")
                ],
            ),
        ]
    )

    session.commit()

    for customer in session.query(Customer):
        for address in customer.addresses:
            print(address)
            # no parent here