在sqlalchemy中,当子表有多个外键到父表时,如何使用多态连接表继承？

Question

我有一个名为的父表pbx_point有一个point_type列.我还有一个名为的子表pbx_route,其中一个列称为point_id指向pbx_point.

我想使用sqlalchemy的连接表继承来通过声明性基础关联这两个表,并使用多态继承

这样可以正常工作 - 或者更确切地说,如果不是以下附加约束,它也会工作:pbx_point还有一个名为initial_route_id指向的外键pbx_route.

我也在下面使用反射,但db就像我上面所描述的那样.我得到的错误是sqlalchemy.exc.AmbiguousForeignKeysError: Can't determine join between 'pbx_point' and 'pbx_route'; tables have more than one foreign key constraint relationship between them. Please specify the 'onclause' of this join explicitly..

这是有道理的,因为"幕后"decarative base正在两个映射类上创建一个relationship()属性.我希望它选择pbx_route.point_id作为parent_id链接,但它也会看到pbx_point.initial_route_id列.如果我正在创建这种关系(),这很容易修复,但我不是 - 声明性继承系统.

我可以传递一个额外的参数__mapper_args__,比如polymorphic_parent_col哪个让我指定我想要的外键？如果没有,我该如何解决这个问题？

谢谢.

class MyBase(DeferredReflection):
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

Base = declarative_base(cls=MyBase)

class pbx_point(Base):
    __mapper_args__ = dict(
        polymorphic_on='point_type',
        with_polymorphic='*',
    )

class pbx_route(pbx_point):
    __mapper_args__ = dict(polymorphic_identity='pbx.route')

这是我得到的堆栈跟踪:

Traceback (most recent call last):
  File "db.py", line 50, in <module>
    Base.prepare(engine)
  File "env/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/api.py", line 431, in prepare
    thingy.map()
  File "env/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 379, in map
    **mapper_args
  File "env/local/lib/python2.7/site-packages/sqlalchemy/orm/__init__.py", line 1147, in mapper
    return Mapper(class_, local_table, *args, **params)
  File "env/local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 213, in __init__
    self._configure_inheritance()
  File "env/local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 517, in _configure_inheritance
    self.local_table)
  File "env/local/lib/python2.7/site-packages/sqlalchemy/sql/util.py", line 397, in join_condition
    "join explicitly." % (a.description, b.description))
sqlalchemy.exc.AmbiguousForeignKeysError: Can't determine join between 'pbx_point' and 'pbx_route'; tables have more than one foreign key constraint relationship between them. Please specify the 'onclause' of this join explicitly.

这表明它正在死于https://bitbucket.org/sqlalchemy/sqlalchemy/src/7f3494ebad58/lib/sqlalchemy/orm/mapper.py?at=default#cl-517.上面的几行使得映射器kwarg得到了回报inherit_condition,这似乎是我需要的.

Answer 1

关键是inherit_condition映射器的参数.多态信息实际上与此步骤无关.

修正后的型号:

class MyBase(DeferredReflection):
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

Base = declarative_base(cls=MyBase)

class pbx_point(Base):
    __mapper_args__ = dict(
        polymorphic_on='point_type',
        with_polymorphic='*',
    )
    id = Column(Integer, primary_key=True)

class pbx_route(pbx_point):
    point_id = Column(Integer, ForeignKey(pbx_point.id))
    __mapper_args__ = dict(
        polymorphic_identity='pbx.route',
        inherit_condition=(point_id == pbx_point.id)
    )

我需要添加id和point_id列以在inherit_condition参数中使用它们.只有使用反射才有可能做到这一点,但这不是一个可怕的障碍.