SQLAlchemy identity map order

insert.py

import random
from sqlalchemy import Column, ForeignKey, Integer, String, create_engine
from sqlalchemy.exc import IntegrityError
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

Base = declarative_base()

class Author(Base):
    __tablename__ = 'author'

    author_id = Column(Integer, primary_key=True)
    name = Column(String(50))

    def __repr__(self):
        return 'Author(%r)' % self.name

class Book(Base):
    __tablename__ = 'book'

    book_id = Column(Integer, primary_key=True)
    title = Column(String(60))
    author_id = Column(Integer, ForeignKey(Author.author_id))

    def __repr__(self):
        return 'Book(%r)' % self.title

def test_insert(object_lists, session):
    for objects in object_lists:
        session.add_all(objects)
        try:
            session.commit()
        except IntegrityError as ex:
            print "Error: %s" % ex
            return False
    return True

if __name__ == "__main__":
    engine = create_engine('mysql://localhost/test')
    session = sessionmaker(bind=engine)()
    Base.metadata.create_all(engine)

    authors = [Author(author_id=i, name='A%d' % i)
               for i in xrange(1, 5)]
    books = [Book(author_id=random.randrange(1, 5), title='B%d' % i)
             for i in xrange(1, 10)]

    # succeeds
    #object_lists = [authors, books]
    # may fail
    #object_lists = [authors + books]
    # may fail
    object_lists = [authors + books]; random.shuffle(object_lists)

    try:
        assert test_insert(object_lists, session)
    finally:
        Base.metadata.drop_all(engine)