Hi!,

I have two entities: Channel and Track. Track has a reference to
Channel (channel field - Track is played on this Channel), and Channel
has a reference to Track (last_track field - last planned Track for
Channel). So definition of tables, classes and mappers is:

"""
import sqlalchemy
from sqlalchemy.orm import *
from sqlalchemy.schema import *
from sqlalchemy.types import *

sqlalchemy_config = {
                     'url'             : 'mysql://...',
                     'echo'            : True,
                     'pool_recycle'    : 600,
                     'convert_unicode' : True,
                    }

ENGINE = sqlalchemy.engine.engine_from_config(sqlalchemy_config, '')
METADATA = sqlalchemy.schema.MetaData()
METADATA.bind = ENGINE
session_maker = sqlalchemy.orm.sessionmaker(autoflush=True,
transactional=True, bind=ENGINE)
SESSION = sqlalchemy.orm.scoped_session(session_maker)

class Channel(object):
    def __init__(self, name):
        self.name = name

class Track(object):
    def __init__(self, title):
        self.title = title

channel_t = Table(
        'channels',
        METADATA,
        Column('id', Integer, primary_key=True),
        Column('name', Unicode(256)),
        Column('last_track_id', Integer, ForeignKey('tracks.id')),
        mysql_engine='InnoDB',
        mysql_charset='utf8'
    )

track_t = Table(
        'tracks',
        METADATA,
        Column('id', Integer, primary_key=True),
        Column('channel_id', Integer, ForeignKey('channels.id'),
nullable=False),
        Column('title', Unicode(256)),
        mysql_engine='InnoDB',
        mysql_charset='utf8'
    )

mapper(Channel, channel_t, properties = {
           'last_track': relation(Track,
primaryjoin=channel_t.c.last_track_id == track_t.c.id),
       }
    )

mapper(Track, track_t, properties={
           'channel': relation(Channel,
primaryjoin=track_t.c.channel_id == channel_t.c.id),
       }
    )

channel1 = SESSION.query(Channel).get(1)
track1 = Track('Track 1')
track1.channel = channel1
channel1.last_track = track1

SESSION.add(track1)
SESSION.commit()
"""

and when I try to commit this, I have following error:
Traceback (most recent call last):
  File "cml.py", line 75, in <module>
    SESSION.commit()
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/orm/scoping.py", line 127, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/orm/session.py", line 671, in commit
    self.transaction.commit()
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/orm/session.py", line 378, in commit
    self._prepare_impl()
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/orm/session.py", line 362, in _prepare_impl
    self.session.flush()
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/orm/session.py", line 1354, in flush
    self._flush(objects)
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/orm/session.py", line 1432, in _flush
    flush_context.execute()
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/orm/unitofwork.py", line 258, in execute
    tasks = self._sort_dependencies()
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/orm/unitofwork.py", line 299, in
_sort_dependencies
    for t in task._sort_circular_dependencies(self,
[self.get_task_by_mapper(i) for i in cycles]):
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/orm/unitofwork.py", line 582, in
_sort_circular_dependencies
    head = topological.sort_as_tree(tuples,
object_to_original_task.iterkeys())
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/topological.py", line 59, in sort_as_tree
    return _organize_as_tree(_sort(tuples, allitems,
allow_cycles=with_cycles))
  File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.5.8-
py2.6.egg/sqlalchemy/topological.py", line 209, in _sort
    raise CircularDependencyError("Circular dependency detected " +
repr(edges) + repr(queue))
sqlalchemy.exc.CircularDependencyError: Circular dependency detected
[(<sqlalchemy.orm.state.InstanceState object at 0x211d050>,
<sqlalchemy.orm.state.InstanceState object at 0x21926d0>),
(<sqlalchemy.orm.state.InstanceState object at 0x21926d0>,
<sqlalchemy.orm.state.InstanceState object at 0x211d050>)][]

what is wrong with this definitions? Documentation says that
CircularDependencyError should appear only when I do topological
sorting.

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