Question

I’m trying to get my head around the “Discriminator on association” example shipped with SQLAlchemy, which defines HasAddresses mixin so each model subclassing HasAddresses magically gets an addresses attribute, which is a collection to which Address objects can be added. The linking is performed through an intermediate table so at the first glance the relationship looks like many-to-many, I hoped to be able to have multiple Addresses linked to a Customer, AND also multiple Customers and Suppliers linked to an Address.

The Address model, however, is set up in such a way that it has a single parent attribute which can only reference a single object. So, in the example, an Address can only be linked to a single Customer or Supplier.

How do I modify that example so Address is able to back-reference multiple parent objects?

Answer 1

we can modify sqlalchemy/examples/generic_associations/table_per_association.py to add a named backref to Address, then a @property which rolls up all backrefs created.

"""table_per_association.py

The HasAddresses mixin will provide a new "address_association" table for
each parent class.   The "address" table will be shared
for 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.ext.declarative import declarative_base, declared_attr
from sqlalchemy import create_engine, Integer, Column, \
                    String, ForeignKey, Table
from sqlalchemy.orm import Session, relationship
import itertools

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)
Base = declarative_base(cls=Base)

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

    This represents all address records in a 
    single table.

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

    @property
    def all_owners(self):
        return list(
            itertools.chain(
            *[
                getattr(self, attr)
                for attr in [a for a in dir(self) if a.endswith("_parents")]
            ]
        ))

    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 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, 
                    backref="%s_parents" % cls.__name__.lower())

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)

a1 = Address(
            street='123 anywhere street',
            city="New York",
            zip="10110")
a2 = Address(
            street='40 main street',
            city="San Francisco",
            zip="95732")

session.add_all([
    Customer(
        name='customer 1', 
        addresses=[a1, a2]
    ),
    Supplier(
        company_name="Ace Hammers",
        addresses=[a1]
    ),
])

session.commit()

for customer in session.query(Customer):
    for address in customer.addresses:
        print address.all_owners