import pkg_resources
pkg_resources.require("SQLAlchemy")
from sqlalchemy import *

metadata = BoundMetaData("postgres://robc@192.168.0.1:5432/sqlalchemy", echo='debug')

table1 = Table('table1', metadata,
               Column('id', Integer, primary_key=True),
               Column('related_id', Integer, ForeignKey('table1.id'), nullable=True),
               Column('type', String),
               )

table2 = Table('table2', metadata,
               Column('id', Integer, ForeignKey('table1.id'), primary_key=True),
               )

metadata.drop_all()
metadata.create_all()

join = polymorphic_union(
    {
    'table2' : table1.join(table2),
    'table1' : table1.select(table1.c.type=='table1'),
    }, None, 'pjoin')

class Table1(object):
    pass

class Table2(object):
    pass

table1_mapper = mapper(Table1, table1,
                       select_table=join,
                       polymorphic_on=join.c.type,
                       polymorphic_identity='table1',
                       properties={'next': relation(Table1, backref='prev', uselist=False, lazy=False)})

table2_mapper = mapper(Table2, table2,
                       inherits=table1_mapper,
                       polymorphic_identity='table2')

Traceback (most recent call last):
  File "bug.py", line 36, in ?
    properties={'next': relation(Table1, backref='prev', uselist=False, lazy=False)})
  File "/usr/lib/python2.4/site-packages/cairo/__init__.py", line 49, in mapper
    
  File "build/bdist.linux-i686/egg/sqlalchemy/orm/mapper.py", line 271, in __init__
  File "build/bdist.linux-i686/egg/sqlalchemy/orm/mapper.py", line 937, in init
  File "build/bdist.linux-i686/egg/sqlalchemy/orm/properties.py", line 242, in do_init
  File "build/bdist.linux-i686/egg/sqlalchemy/orm/properties.py", line 427, in do_init_subclass
  File "build/bdist.linux-i686/egg/sqlalchemy/orm/properties.py", line 329, in do_init_subclass
  File "build/bdist.linux-i686/egg/sqlalchemy/orm/mapper.py", line 848, in query
  File "build/bdist.linux-i686/egg/sqlalchemy/orm/query.py", line 28, in __init__
KeyError: <sqlalchemy.sql.Alias object at 0xa78a5cac>