跨文件的SQLAlchemy类

Question

我试图弄清楚如何将SQLAlchemy类分布到多个文件中，而我可以终生不搞清楚如何做到这一点。我是SQLAlchemy的新手，如果这个问题不重要，请原谅我。

在 各自的文件中 考虑以下三个类：

A.py：

from sqlalchemy import *
from main import Base

class A(Base):
    __tablename__ = "A"
    id  = Column(Integer, primary_key=True)
    Bs  = relationship("B", backref="A.id")
    Cs  = relationship("C", backref="A.id")

B.py：

from sqlalchemy import *
from main import Base

class B(Base):
    __tablename__ = "B"
    id    = Column(Integer, primary_key=True)
    A_id  = Column(Integer, ForeignKey("A.id"))

C.py：

from sqlalchemy import *
from main import Base

class C(Base):
    __tablename__ = "C"    
    id    = Column(Integer, primary_key=True)
    A_id  = Column(Integer, ForeignKey("A.id"))

然后说我们有一个 main.py之类的 东西：

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, backref, sessionmaker

Base = declarative_base()

import A
import B
import C

engine = create_engine("sqlite:///test.db")
Base.metadata.create_all(engine, checkfirst=True)
Session = sessionmaker(bind=engine)
session = Session()

a  = A.A()
b1 = B.B()
b2 = B.B()
c1 = C.C()
c2 = C.C()

a.Bs.append(b1)
a.Bs.append(b2)    
a.Cs.append(c1)
a.Cs.append(c2)    
session.add(a)
session.commit()

上面给出了错误：

sqlalchemy.exc.NoReferencedTableError: Foreign key assocated with column 'C.A_id' could not find table 'A' with which to generate a foreign key to target column 'id'

如何在这些文件之间共享声明式库？

考虑到我可能 还会 在上面 放置 诸如 Pylons 或 Turbogears之 类的东西，完成此任务的“正确”方法是什么？

编辑10-03-2011

我从Pyramids框架中找到了这个描述，该描述描述了问题，更重要的是 验证
了这是一个实际问题，而不仅仅是（只是）我那困惑的自我。希望它可以帮助其他敢于走这条危险道路的人:)

Answer 1

解决您的问题的最简单方法是Base取出导入的模块A，B然后C；中断循环导入。

base.py

from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()

py

from sqlalchemy import *
from base import Base
from sqlalchemy.orm import relationship

class A(Base):
    __tablename__ = "A"
    id  = Column(Integer, primary_key=True)
    Bs  = relationship("B", backref="A.id")
    Cs  = relationship("C", backref="A.id")

b.py

from sqlalchemy import *
from base import Base

class B(Base):
    __tablename__ = "B"
    id    = Column(Integer, primary_key=True)
    A_id  = Column(Integer, ForeignKey("A.id"))

py

from sqlalchemy import *
from base import Base

class C(Base):
    __tablename__ = "C"    
    id    = Column(Integer, primary_key=True)
    A_id  = Column(Integer, ForeignKey("A.id"))

main.py

from sqlalchemy import create_engine
from sqlalchemy.orm import relationship, backref, sessionmaker

import base


import a
import b
import c

engine = create_engine("sqlite:///:memory:")
base.Base.metadata.create_all(engine, checkfirst=True)
Session = sessionmaker(bind=engine)
session = Session()

a1 = a.A()
b1 = b.B()
b2 = b.B()
c1 = c.C()
c2 = c.C()

a1.Bs.append(b1)
a1.Bs.append(b2)    
a1.Cs.append(c1)
a1.Cs.append(c2)    
session.add(a1)
session.commit()

适用于我的机器：

$ python main.py ; echo $?
0