SQLAlchemy declarativo error uno a muchos no definido

Question

Estoy tratando de imaginar cómo definir una relación de uno a muchos usando el ORM declarativo de SQLAlchemy, y tratando de hacer que the example funcione, pero me da un error que mi subclase no se puede encontrar (naturalmente, porque se declaró más tarde ...)

InvalidRequestError: When initializing mapper Mapper|Parent|parent, expression 'Child' failed to locate a name ("name 'Child' is not defined"). If this is a class name, consider adding this relationship() to the class after both dependent classes have been defined.

Pero ¿Cómo defino esto, sin el error?

El código:

from sqlalchemy import create_engine 
from sqlalchemy import Column, Integer, ForeignKey 
from sqlalchemy.ext.declarative import declarative_base 
from sqlalchemy.orm import sessionmaker, relationship 
from dev.historyMeta import VersionedMeta, VersionedListener 

global engine, Base, Session 
engine = create_engine('mysql+mysqldb://user:pass@localhost:3306/testdb', pool_recycle=3600) 
Base = declarative_base(bind=engine, metaclass=VersionedMeta) 
Session = sessionmaker(extension=VersionedListener()) 


class Parent(Base): 
    __tablename__ = 'parent' 
    id = Column(Integer, primary_key=True) 
    children = relationship("Child", backref="parent") 

class Child(Base): 
    __tablename__ = 'child' 
    id = Column(Integer, primary_key=True) 
    parent_id = Column(Integer, ForeignKey('parent.id')) 

Base.metadata.create_all() 

Answer 1

Así es como lo hago:

from sqlalchemy import create_engine 
from sqlalchemy import Column, Integer, ForeignKey 
from sqlalchemy.ext.declarative import declarative_base 
from sqlalchemy.orm import sessionmaker, relationship 

engine = create_engine('sqlite://', echo=True) 
Base = declarative_base(bind=engine) 
Session = sessionmaker(bind=engine) 


class Parent(Base): 
    __tablename__ = 'parent' 
    id = Column(Integer, primary_key=True) 

class Child(Base): 
    __tablename__ = 'child' 
    id = Column(Integer, primary_key=True) 
    parent_id = Column(Integer, ForeignKey('parent.id')) 
    parent = relationship(Parent, backref='children') 

Base.metadata.create_all()