examples.generic_associations.discriminator_on_association — SQLAlchemy 2.0 Documentation

Release: 2.0.50 | Release Date: May 24, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.generic_associations.discriminator_on_association

    """Illustrates a mixin which provides a generic association
    using a single target table and a single association table,
    referred to by all parent tables.  The association table
    contains a "discriminator" column which determines what type of
    parent object associates to each particular row in the association
    table.

    SQLAlchemy's single-table-inheritance feature is used
    to target different association types.

    This configuration attempts to simulate a so-called "generic foreign key"
    as closely as possible without actually foregoing the use of real
    foreign keys.   Unlike table-per-related and table-per-association,
    it uses a fixed number of tables to serve any number of potential parent
    objects, but is also slightly more complex.

    """

    from sqlalchemy import create_engine
    from sqlalchemy import ForeignKey
    from sqlalchemy.ext.associationproxy import association_proxy
    from sqlalchemy.orm import backref
    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 AddressAssociation(Base):
        """Associates a collection of Address objects
        with a particular parent.
        """

        __tablename__ = "address_association"

        discriminator: Mapped[str] = mapped_column()
        """Refers to the type of parent."""
        addresses: Mapped[list["Address"]] = relationship(
            back_populates="association"
        )

        __mapper_args__ = {"polymorphic_on": discriminator}

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

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

        association_id: Mapped[int] = mapped_column(
            ForeignKey("address_association.id")
        )
        street: Mapped[str]
        city: Mapped[str]
        zip: Mapped[str]
        association: Mapped["AddressAssociation"] = relationship(
            back_populates="addresses"
        )

        parent = association_proxy("association", "parent")

        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 relationship to
        the address_association table for each parent.
        """

        @declared_attr
        def address_association_id(cls) -> Mapped[int]:
            return mapped_column(ForeignKey("address_association.id"))

        @declared_attr
        def address_association(cls):
            name = cls.__name__
            discriminator = name.lower()

            assoc_cls = type(
                f"{name}AddressAssociation",
                (AddressAssociation,),
                dict(
                    __tablename__=None,
                    __mapper_args__={"polymorphic_identity": discriminator},
                ),
            )

            cls.addresses = association_proxy(
                "address_association",
                "addresses",
                creator=lambda addresses: assoc_cls(addresses=addresses),
            )
            return relationship(
                assoc_cls, backref=backref("parent", uselist=False)
            )

    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)
            print(address.parent)