examples.postgis.generic_fk
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2023-12-01
"""Illustrates a so-called "generic foreign key", in a similar fashion to that of popular frameworks such as Django, ROR, etc. This approach bypasses standard referential integrity practices, in that the "foreign key" column is not actually constrained to refer to any particular table; instead, in-application logic is used to determine which table is referenced. This approach is not in line with SQLAlchemy's usual style, as foregoing foreign key integrity means that the tables can easily contain invalid references and also have no ability to use in-database cascade functionality. However, due to the popularity of these systems, as well as that it uses the fewest number of tables (which doesn't really offer any "advantage", though seems to be comforting to many) this recipe remains in high demand, so in the interests of having an easy StackOverflow answer queued up, here it is. The author recommends "table_per_related" or "table_per_association" instead of this approach. .. versionadded:: 0.8.3 """ from sqlalchemy import and_ from sqlalchemy import Column from sqlalchemy import create_engine from sqlalchemy import event 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 backref from sqlalchemy.orm import foreign from sqlalchemy.orm import relationship from sqlalchemy.orm import remote 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 Address(Base): """The Address class. This represents all address records in a single table. """ street = Column(String) city = Column(String) zip = Column(String) discriminator = Column(String) """Refers to the type of parent.""" parent_id = Column(Integer) """Refers to the primary key of the parent. This could refer to any table. """ @property def parent(self): """Provides in-Python access to the "parent" by choosing the appropriate relationship. """ return getattr(self, "parent_%s" % self.discriminator) 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. """ @event.listens_for(HasAddresses, "mapper_configured", propagate=True) def setup_listener(mapper, class_): name = class_.__name__ discriminator = name.lower() class_.addresses = relationship( Address, primaryjoin=and_( class_.id == foreign(remote(Address.parent_id)), Address.discriminator == discriminator, ), backref=backref( "parent_%s" % discriminator, primaryjoin=remote(class_.id) == foreign(Address.parent_id), ), ) @event.listens_for(class_.addresses, "append") def append_address(target, value, initiator): value.discriminator = discriminator 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)