Bug in sqla hstore mutability implementation

sqla_hstore_update_bug.py

from sqlalchemy import create_engine
from sqlalchemy import Column, Integer
from sqlalchemy.dialects.postgres import HSTORE
from sqlalchemy.sql import text
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('postgresql:///hstore_test', echo=True)
session = sessionmaker(bind=engine)()
Base = declarative_base()

class C(Base):
    __tablename__ = 'foo'

    id = Column(Integer, primary_key=True)
    hashes = Column(HSTORE, server_default=text("''::hstore"))

    def __init__(self):
        self.hashes = {}

    def __repr__(self):
        return '%s: %s' % (self.id, self.hashes)

Base.metadata.create_all(engine)

session.query(C).delete()

a = C()
a.hashes["foo"] = '123'

b = C()
b.hashes = {'foo': '123'}

session.add_all([a, b])
session.commit()

a, b = session.query(C).all()
print a.hashes['foo'], b.hashes['foo']

a.hashes['foo'] = "123456"
b.hashes = {"foo": '123456'}

session.commit()

a, b = session.query(C).all()
assert a.hashes['foo'] == b.hashes['foo']