examples.generic_associations.table_per_related — SQLAlchemy 2.0 Documentation

Release: 2.0.39 | Release Date: March 11, 2025

# SQLAlchemy 2.0 Documentation

# Source code for examples.generic_associations.table_per_related

    """Illustrates a generic association which persists association
    objects within individual tables, each one generated to persist
    those objects on behalf of a particular parent class.

    This configuration has the advantage that each type of parent
    maintains its "Address" rows separately, so that collection
    size for one type of parent will have no impact on other types
    of parent.   Navigation between parent and "Address" is simple,
    direct, and bidirectional.

    This recipe is the most efficient (speed wise and storage wise)
    and simple of all of them.

    The creation of many related tables may seem at first like an issue
    but there really isn't any - the management and targeting of these tables
    is completely automated.

    """

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

    @as_declarative()
    class Base:
        """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 Address:
        """Define columns that will be present in each
        'Address' table.

        This is a declarative mixin, so additional mapped
        attributes beyond simple columns specified here
        should be set up using @declared_attr.

        """

        street = Column(String)
        city = Column(String)
        zip = Column(String)

        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 class
        for each parent.

        """

        @declared_attr
        def addresses(cls):
            cls.Address = type(
                "%sAddress" % cls.__name__,
                (Address, Base),
                dict(
                    __tablename__="%s_address" % cls.__tablename__,
                    parent_id=Column(
                        Integer, ForeignKey("%s.id" % cls.__tablename__)
                    ),
                    parent=relationship(cls),
                ),
            )
            return relationship(cls.Address)

    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=[
                    Customer.Address(
                        street="123 anywhere street", city="New York", zip="10110"
                    ),
                    Customer.Address(
                        street="40 main street", city="San Francisco", zip="95732"
                    ),
                ],
            ),
            Supplier(
                company_name="Ace Hammers",
                addresses=[
                    Supplier.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)