I'm hoping someone can clarify my understanding of the Session's
identity map. I've created an example to frame my question:
>>> import psycopg2
>>> psycopg2.__version__
'2.2.2 (dt dec ext pq3)'
>>> import sqlalchemy
>>> sqlalchemy.__version__
'0.6.4'
>>> from sqlalchemy import create_engine, MetaData, Table, Column, Integer,
>>> String
>>> from sqlalchemy.orm import mapper, sessionmaker
>>> engine = create_engine('postgresql+psycopg2://ddt:d...@localhost/ddt')
>>> metadata = MetaData(engine)
>>> t = Table('t', metadata,
... Column('i', Integer, primary_key=True),
... Column('s', String(10)))
... >>>
>>> metadata.create_all()
>>> class T(object): pass
... >>>
>>> mapper(T, t)
>>> Session = sessionmaker(bind=engine)
>>> session = Session(autocommit=True, autoflush=True, expire_on_commit=True)
Now In psql:
insert into t values (1, 'a');
insert into t values (2, 'b');
>>> [(t.i, t.s) for t in session.query(T)]
[(1, u'a'), (2, u'b')]
>>> session.identity_map
{(<class '__main__.T'>, (2,)): <sqlalchemy.orm.state.InstanceState
object at 0x1c47a90>}
The identity_map is holding onto the tail of this list comprehension.
This leads some really weird results:
In psql:
update t set s='c';
>>> [(t.i, t.s) for t in session.query(T)]
[(1, u'c'), (2, u'b')]
As an end-user, I would either expect both items to be updated or
neither of them to be updated. Why is only the last item being held in
the identity map?
Thanks in advance!
Graham
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