SQLAlchemy declarative one-to-many not defined error

Question

I'm trying to figure how to define a one-to-many relationship using SQLAlchemy's declarative ORM, and trying to get the example to work, but I'm getting an error that my sub-class can't be found (naturally, because it's declared later...)

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.

But how do I define this, without the error?

The code:

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

Here's how I do it:

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()