NickPerkins wrote:
> Just when I thought I understood the Session object, I found this
> behavior:
> ( I am starting to think about concurrency and locking )
>
> I create 2 sessions, and load each with the same ( persisted ) object.
> The objects appear to be independent, even after both sessions have
> flushed,
> but when I commit one session, it suddenly picks up the change from
> the other session!
>
> This is confusing me!
> What is going on here?
> ( my business object is a "Policy", and column "test" is not the PK )
>
>
> class PolicyNotFoundException(Exception):
> pass
>
> def open_policy(key):
> session = Session()
> try:
> policy = session.query(Policy).filter_by(key=key).all()[0]
> return policy, session
> except:
> raise PolicyNotFoundException
>
> def test_concurrent_updates():
> policy1,session1 = open_policy('NICK')
> policy2,session2 = open_policy('NICK') # retrieves on pk...gets
> same record
> assert( policy1 != policy2 )
> assert( session1 != session2 )
> assert( policy1 in session1 )
> assert( policy1 not in session2 )
> assert( policy2 in session2 )
> assert( policy2 not in session1 )
>
> policy1.test = 'ONE'
> session1.flush()
>
> policy2.test = 'TWO'
> session2.flush()
>
> assert( policy1.test == 'ONE' ) # not affected by flush of session
> 2
>
> session1.commit()
> assert( policy1.test == 'TWO' ) # really?
>
I see two possible explanations:
1. (More likely) You are using a database that does not support
transactions (e.g. MySQL with MyISAM tables). If this is the case,
every insert/update/delete is immediately visible to other DB
connections. In this case, assert(policy1.test == 'ONE') succeeds
because the policy1 object has a cached value for its 'test'
attribute. If you had put session1.expire(policy1)) between
session2.flush() and assert(policy1.test == 'ONE'), the assertion
would fail. Since session1.commit() expires every object in
session1, policy1 will then pick up the new value from the DB.
2. Your sessions have autocommit enabled on your sessions. I think
SQLAlchemy would auto-expire objects in this case (causing
assert(policy1.test == 'ONE') to fail), but I'm not sure.
I would recommend that you enable SQL logging on your engine so you can
observe exactly when policy1 fetches the new values from the DB, and
when COMMITs are getting issued to the DB.
-Conor
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