import traceback

from sqlalchemy import Column, Integer, String, ForeignKey, Table
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy

Base = declarative_base()

class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    name = Column(String(64))
    kws = relationship("Keyword", secondary='user_keyword')

    keywords = association_proxy('kws', 'keyword')


class UserKeyword(Base):
    __tablename__ = 'user_keyword'
    id = Column(Integer, primary_key=True)
    user_id = Column(Integer, ForeignKey('user.id'))
    keyword_id = Column(Integer, ForeignKey('keyword.id'))


class Keyword(Base):
    __tablename__ = 'keyword'
    id = Column(Integer, primary_key=True)
    keyword = Column('keyword', String(64))


def main():

    print 'owning_class is None'
    assert User.__mapper__.all_orm_descriptors['keywords'].owning_class is None

    print 'association proxy throwing error'
    try:
        User.__mapper__.all_orm_descriptors['keywords'].target_class
    except Exception:
        print '-'*50
        traceback.print_exc()
        print '-'*50

    print 'accessing association proxy via class sets owning_class'
    assert User.keywords is not None

    print 'owning_class remains set'
    assert User.__mapper__.all_orm_descriptors['keywords'].owning_class is not  None

    print 'association proxy no longer throwing error'
    User.__mapper__.all_orm_descriptors['keywords'].target_class


if __name__ == '__main__':
    main()