All of the crazy mappings today are blowing my mind, so I'll point you to an old unit test with a doubly linked list:
http://www.sqlalchemy.org/trac/browser/sqlalchemy/trunk/test/orm/inheritance/poly_linked_list.py the above uses just a single foreign key (but we can still traverse bi- directionally of course). Im not sure what the "double" foreign key approach here is for. wouldnt the "previous" task of one task be the "next" task for another with just one FK relation ? in the relational model, a single FK is automatically bidirectional, unlike a reference in a programming language. On Jan 11, 2008, at 11:57 AM, Jonathan LaCour wrote: > > Jonathan LaCour wrote: > >>> I am attempting to model a doubly-linked list, as follows: >> >> ... seems to do the trick. I had tried using backref's earlier, >> but it was failing because I was specifying a "remote_side" >> keyword argument to the backref(), which was making it blow up >> with cycle detection exceptions for some reason. > > Oops, spoke too soon! Here is a test case which shows something > quite odd. I create some elements, link them together, and then > walk the relations forward and backward, printing out the results. > All seems fine. Then, I update the order of the linked list, and > print them out forward, and they work okay, but when I print things > out in reverse order, its all screwy. > > Any ideas? > > -------------------------------------------------------------------- > > from sqlalchemy import * > from sqlalchemy.orm import * > > engine = create_engine('sqlite:///') > metadata = MetaData(engine) > Session = scoped_session( > sessionmaker(bind=engine, autoflush=True, transactional=True) > ) > > > task_table = Table('task', metadata, > Column('id', Integer, primary_key=True), > Column('name', Unicode), > Column('next_task_id', Integer, ForeignKey('task.id')), > Column('previous_task_id', Integer, ForeignKey('task.id')) > ) > > > class Task(object): > def __init__(self, **kw): > for key, value in kw.items(): > setattr(self, key, value) > > def __repr__(self): > return '<Task :: %s>' % self.name > > Session.mapper(Task, task_table, properties={ > 'next_task' : relation( > Task, > primaryjoin=task_table.c.next_task_id==task_table.c.id, > uselist=False, > remote_side=task_table.c.id, > backref=backref( > 'previous_task', > primaryjoin=task_table.c.previous_task_id==task_table.c.id, > uselist=False > ) > ), > }) > > > if __name__ == '__main__': > metadata.create_all() > > t1 = Task(name=u'Item One') > t2 = Task(name=u'Item Two') > t3 = Task(name=u'Item Three') > t4 = Task(name=u'Item Four') > t5 = Task(name=u'Item Five') > t6 = Task(name=u'Item Six') > > t1.next_task = t2 > t2.next_task = t3 > t3.next_task = t4 > t4.next_task = t5 > t5.next_task = t6 > > Session.commit() > Session.clear() > > print '-' * 80 > task = Task.query.filter_by(name=u'Item One').one() > while task is not None: > print task > task = task.next_task > print '-' * 80 > > print '-' * 80 > task = Task.query.filter_by(name=u'Item Six').one() > while task is not None: > print task > task = task.previous_task > print '-' * 80 > > > Session.clear() > > t1 = Task.query.filter_by(name=u'Item One').one() > t2 = Task.query.filter_by(name=u'Item Two').one() > t3 = Task.query.filter_by(name=u'Item Three').one() > t4 = Task.query.filter_by(name=u'Item Four').one() > t5 = Task.query.filter_by(name=u'Item Five').one() > t6 = Task.query.filter_by(name=u'Item Six').one() > > t1.next_task = t5 > t5.next_task = t2 > t4.next_task = t6 > > Session.commit() > Session.clear() > > print '-' * 80 > task = Task.query.filter_by(name=u'Item One').one() > while task is not None: > print task > task = task.next_task > print '-' * 80 > > print '-' * 80 > task = Task.query.filter_by(name=u'Item Six').one() > while task is not None: > print task > task = task.previous_task > print '-' * 80 > > -------------------------------------------------------------------- > > -- > Jonathan LaCour > http://cleverdevil.org > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to sqlalchemy@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en -~----------~----~----~----~------~----~------~--~---
