Foreign keys in object mixins

output

Traceback (most recent call last):
  File "sqla_mixin.py", line 45, in <module>
    DeclarativeBase.metadata.create_all(engine)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/schema.py", line 2415, in create_all
    tables=tables)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/engine/base.py", line 2178, in _run_visitor
    conn._run_visitor(visitorcallable, element, **kwargs)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/engine/base.py", line 1857, in _run_visitor
    **kwargs).traverse_single(element)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/sql/visitors.py", line 86, in traverse_single
    return meth(obj, **kw)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/engine/ddl.py", line 65, in visit_metadata
    self.traverse_single(table, create_ok=True)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/sql/visitors.py", line 86, in traverse_single
    return meth(obj, **kw)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/engine/ddl.py", line 82, in visit_table
    self.connection.execute(schema.CreateTable(table))
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/engine/base.py", line 1358, in execute
    params)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/engine/base.py", line 1443, in _execute_ddl
    compiled = ddl.compile(dialect=dialect)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/sql/expression.py", line 1643, in compile
    return self._compiler(dialect, bind=bind, **kw)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/schema.py", line 2752, in _compiler
    return dialect.ddl_compiler(dialect, self, **kw)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/engine/base.py", line 699, in __init__
    self.string = self.process(self.statement)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/engine/base.py", line 718, in process
    return obj._compiler_dispatch(self, **kwargs)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/sql/visitors.py", line 59, in _compiler_dispatch
    return getter(visitor)(self, **kw)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/sql/compiler.py", line 1278, in visit_create_table
    const = self.create_table_constraints(table)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/sql/compiler.py", line 1298, in create_table_constraints
    for constraint in constraints 
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/sql/compiler.py", line 1296, in <genexpr>
    return ", \n\t".join(p for p in
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/sql/compiler.py", line 1304, in <genexpr>
    not getattr(constraint, 'use_alter', False)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/engine/base.py", line 718, in process
    return obj._compiler_dispatch(self, **kwargs)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/sql/visitors.py", line 59, in _compiler_dispatch
    return getter(visitor)(self, **kw)
  File "/home/plq/src/sqlalchemy/lib/sqlalchemy/dialects/sqlite/base.py", line 377, in visit_foreign_key_constraint
    if local_table.schema != remote_table.schema:
AttributeError: 'NoneType' object has no attribute 'schema'

sqla_mixin.py

#!/usr/bin/python

from sqlalchemy import Integer
from sqlalchemy import UniqueConstraint
from sqlalchemy import ForeignKey
from sqlalchemy import Column
from sqlalchemy import Unicode
from sqlalchemy import Boolean

from sqlalchemy.orm import mapper
from sqlalchemy.engine import create_engine
from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy.ext.declarative import declarative_base

DeclarativeBase = declarative_base()

class DomainMixin(object):
    id = Column(Integer, primary_key=True)

    domain_name = Column(Unicode(255), unique=True)
    active = Column(Boolean)

class UserMixin(object):
    __table_args__ = (
        UniqueConstraint('user_name', 'domain_id'),
    )

    id = Column(Integer, primary_key=True)

    @declared_attr
    def domain_id(cls):
        return Column(Integer, ForeignKey(DomainMixin.id))

    user_name = Column(Unicode(64))

class Domain(DeclarativeBase, DomainMixin):
    __tablename__='domain'

class User(DeclarativeBase, UserMixin):
    __tablename__='user'

if __name__ == '__main__':
    engine = create_engine('sqlite:///:memory:')
    DeclarativeBase.metadata.create_all(engine)