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 Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import String
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import as_declarative
from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy.orm import backref
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session


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

    """

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

    id = Column(Integer, primary_key=True)


class AddressAssociation(Base):
    """Associates a collection of Address objects
    with a particular parent.

    """

    __tablename__ = "address_association"

    discriminator = Column(String)
    """Refers to the type of parent."""

    __mapper_args__ = {"polymorphic_on": discriminator}


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

    This represents all address records in a
    single table.

    """

    association_id = Column(Integer, ForeignKey("address_association.id"))
    street = Column(String)
    city = Column(String)
    zip = Column(String)
    association = relationship("AddressAssociation", backref="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(object):
    """HasAddresses mixin, creates a relationship to
    the address_association table for each parent.

    """

    @declared_attr
    def address_association_id(cls):
        return Column(Integer, ForeignKey("address_association.id"))

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

        assoc_cls = type(
            "%sAddressAssociation" % name,
            (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 = Column(String)


class Supplier(HasAddresses, Base):
    company_name = Column(String)


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)