Test case for implicit merge in SQLAlchemy when PK is set in transient object

gistfile1.py

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

Base = declarative_base()

class Parent(Base):
    __tablename__ = 'parents'
    id = Column(Integer, primary_key=True)
    #children = relationship('Child', cascade='all') # doesn't merge
    children = relationship('Child', cascade='all,delete-orphan')
    def __repr__(self):
        return 'Parent(id=%r, children=%r)' % (self.id, self.children)

class Child(Base):
    __tablename__ = 'children'
    id = Column(Integer, primary_key=True)
    parent_id = Column(ForeignKey(Parent.id), nullable=False)
    parent = relationship(Parent)
    data1 = Column(Integer, server_default='1', default=2)
    data2 = Column(Integer, server_default='1', default=2)
    def __repr__(self):
        return 'Child(id=%r, data1=%r, data2=%r)' % (
                            self.id, self.data1, self.data2)

engine = create_engine('sqlite://', echo=True)
Base.metadata.create_all(engine)
db = sessionmaker(bind=engine)()

p = Parent(children=[Child(data1=3, data2=3)])
db.add(p)
db.commit()
print p
p_id = p.id
c_id = p.children[0].id

db.expunge_all()
p = db.query(Parent).get(p_id)
#print p # Access to children might affect behavior 
p.children = [Child(id=c_id, data1=None)]
db.commit()
db.expunge_all()
p = db.query(Parent).get(p_id)
print p