sqlalchemy OrderingList issue

issueorderinglist.py

# -*- coding: utf-8 -*-

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

sa_engine = create_engine("sqlite:///:memory:")

Session = sessionmaker(bind=sa_engine)
session = Session()

Base = declarative_base()

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship('Child', backref='parent',
                            collection_class=ordering_list('position'),
                            order_by='Child.position'
                            )

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

    def __repr__(self):
        return "<Child id={0}, name={1}, position={2}>".format(self.id, self.name, self.position)

Base.metadata.create_all(sa_engine)


p = Parent()
session.add(p)

c1 = Child(name="aaa")
session.add(c1)

c2 = Child(name="ccc")
session.add(c2)

p.children.append(c1)
p.children.append(c2)

session.commit()

[parent] = session.query(Parent).all()
print parent.children
# -> [<Child id=1, name=aaa, position=0>, <Child id=2, name=ccc, position=1>]

c3 = Child(name="bbb")
session.add(c3)

parent.children.insert(1, c3)
print parent.children
# -> [<Child id=1, name=aaa, position=0>, <Child id=None, name=bbb, position=1>, <Child id=2, name=ccc, position=2>]

session.commit()
[parent] = session.query(Parent).all()

# Fixed, we keep now the children positions after a commit.
print parent.children
# -> [<Child id=1, name=aaa, position=0>, <Child id=3, name=bbb, position=1>, <Child id=2, name=ccc, position=2>]