python - SQLAlchemy classes across files

Question

I'm trying to figure out how to have SQLAlchemy classes spread across several files, and I can for my life not figure out how to do it. I am pretty new to SQLAlchemy so forgive me if this question is trivial..

Consider these 3 classes in each their own file:

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"))

And then say we have a main.py something like this:

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()

The above gives the error:

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'

How do I share the declarative base across these files?

What is the "the right" way to accomplish this, considering that I might throw something like Pylons or Turbogears on top of this?

edit 10-03-2011

I found this description from the Pyramids framework which describes the problem and more importantly verifies that this is an actual issue and not (only) just my confused self that's the problem. Hope it can help others who dares down this dangerous road :)

Answer 1

The simplest solution to your problem will be to take Base out of the module that imports A, B and C; Break the cyclic import.

base.py

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

a.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"))

c.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()

Works on my machine:

$ python main.py ; echo $?
0