Question

我对SQLAlchemy很新，甚至是数据库编程，也许我的问题太简单了。现在我有两个类/表：

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String(40))
    ...

class Computer(Base):
    __tablename__ = 'comps'
    id = Column(Integer, primary_key=True)
    buyer_id = Column(None, ForeignKey('users.id'))
    user_id = Column(None, ForeignKey('users.id'))
    buyer = relation(User, backref=backref('buys', order_by=id))
    user = relation(User, backref=backref('usings', order_by=id))

当然，它无法运行。这是回溯：

  File "/Library/Python/2.6/site-packages/SQLAlchemy-0.5.8-py2.6.egg/sqlalchemy/orm/state.py", line 71, in initialize_instance
    fn(self, instance, args, kwargs)
  File "/Library/Python/2.6/site-packages/SQLAlchemy-0.5.8-py2.6.egg/sqlalchemy/orm/mapper.py", line 1829, in _event_on_init
    instrumenting_mapper.compile()
  File "/Library/Python/2.6/site-packages/SQLAlchemy-0.5.8-py2.6.egg/sqlalchemy/orm/mapper.py", line 687, in compile
    mapper._post_configure_properties()
  File "/Library/Python/2.6/site-packages/SQLAlchemy-0.5.8-py2.6.egg/sqlalchemy/orm/mapper.py", line 716, in _post_configure_properties
    prop.init()
  File "/Library/Python/2.6/site-packages/SQLAlchemy-0.5.8-py2.6.egg/sqlalchemy/orm/interfaces.py", line 408, in init
    self.do_init()
  File "/Library/Python/2.6/site-packages/SQLAlchemy-0.5.8-py2.6.egg/sqlalchemy/orm/properties.py", line 716, in do_init
    self._determine_joins()
  File "/Library/Python/2.6/site-packages/SQLAlchemy-0.5.8-py2.6.egg/sqlalchemy/orm/properties.py", line 806, in _determine_joins
    "many-to-many relation, 'secondaryjoin' is needed as well." % (self))
sqlalchemy.exc.ArgumentError: Could not determine join condition between parent/child tables on relation Package.maintainer.  Specify a 'primaryjoin' expression.  If this is a many-to-many relation, 'secondaryjoin' is needed as well.

类Computer中有两个外键，因此relation（）调用无法确定应该使用哪一个。我想我必须使用额外的参数来指定它，对吧？怎么样？感谢

Answer 1

正确的语法应该是：

buyer = relation(User, backref=backref('buys', order_by=id))
user = relation(User, backref=backref('usings', order_by=id))

P.S。下次请通过发布追溯来指定“无法运行”的含义。

更新：更新后的问题中的回溯确切地说明了您的需求：指定primaryjoin条件：

buyer = relation(User, primaryjoin=(buyer_id==User.id),
                 backref=backref('buys', order_by=id))
user = relation(User, primaryjoin=(user_id==User.id),
                backref=backref('usings', order_by=id))

声明式样式SQLAlchemy关系的初始问题（）

1 个答案: