聲明式風格的入門者問題SQLAlchemy關係（）

Question

我對SQLAlchemy甚至數據庫編程頗爲陌生，也許我的問題太簡單了。 現在我有兩個班/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)) 

當然，它不能運行。這是回溯：

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. 

在Computer類中有兩個外鍵，所以relation（）調用無法確定應該使用哪一個。我想我必須使用額外的參數來指定它，對吧？ 和howto？由於

Answer 1

正確的語法應爲：

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

附：下次請通過發佈回溯來指定「無法運行」的含義。

更新：在更新的問題回溯說，正是你需要的：指定primaryjoin條件：

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))