SQLAlchemy声明性具体的自动加载表继承

我已经有一个数据库,希望使用sqlacalchemy访问它。因为,数据库结构是由另一段代码(实际上是django-orm)管理的,我不想重复,描述我正在使用的每个表结构 autoload 反省。我只能继承一个简单的具体表。

Payment                FooPayment
  + id (PK) <----FK------+ payment_ptr_id (PK)
  + user_id              + foo
  + amount
  + date

下面是代码,表SQL描述为docstrings:

class Payment(Base):
    """
    CREATE TABLE payments(
      id serial NOT NULL,
      user_id integer NOT NULL,
      amount numeric(11,2) NOT NULL,
      date timestamp with time zone NOT NULL,
      CONSTRAINT payment_pkey PRIMARY KEY (id),
      CONSTRAINT payment_user_id_fkey FOREIGN KEY (user_id)
          REFERENCES users (id) MATCH SIMPLE)
    """
    __tablename__ = 'payments'
    __table_args__ = {'autoload': True}
    # user = relation(User)

class FooPayment(Payment):
    """
    CREATE TABLE payments_foo(
      payment_ptr_id integer NOT NULL,
      foo integer NOT NULL,
      CONSTRAINT payments_foo_pkey PRIMARY KEY (payment_ptr_id),
      CONSTRAINT payments_foo_payment_ptr_id_fkey
          FOREIGN KEY (payment_ptr_id)
          REFERENCES payments (id) MATCH SIMPLE)
    """
    __tablename__ = 'payments_foo'
    __table_args__ = {'autoload': True}
    __mapper_args__ = {'concrete': True}

实际的表有额外的列,但这与问题完全无关,因此为了尽量减少代码,我已经将所有内容简化到核心。

问题是,当我运行此命令时:

payment = session.query(FooPayment).filter(Payment.amount >= 200.0).first()
print payment.date

结果SQL毫无意义(请注意,缺少连接条件):

SELECT payments_foo.payment_ptr_id AS payments_foo_payment_ptr_id,
       ... /* More `payments_foo' columns and NO columns from `payments' */
    FROM payments_foo, payments
    WHERE payments.amount >= 200.0 LIMIT 1 OFFSET 0

当我试图进入 payment.date 我得到以下错误: Concrete Mapper|FooPayment|payments_foo does not implement attribute u'date' at the instance level.

我尝试添加隐式外键引用 id = Column('payment_ptr_id', Integer, ForeignKey('payments_payment.id'), primary_key=True) 到 FooPayment 没有任何成功。尝试 print session.query(Payment).first().user 工作(我省略了 User 课堂和评论的话)完美,所以FK反思工作。

如何执行简单查询 诱骗 和访问 Payment 结果实例的值?

我使用的是sqlacalchemy 0.5.3、postgresql 8.3、psycopg2和python 2.5.2。 谢谢你的建议。