On Thu, Jan 17, 2013 at 3:54 PM, Michael Bayer <mike...@zzzcomputing.com>wrote:
>
> On Jan 17, 2013, at 6:45 PM, Ken Lareau wrote:
>
> Sadly, it looks like when I try the code in my full application, it is
> failing miserably.
> I'm seeing errors like this:
>
> sqlalchemy.orm.exc.DetachedInstanceError: Instance <Deployments at
> 0x3367910> is not bound to a Session; attribute refresh operation cannot
> proceed
>
> and
>
> sqlalchemy.exc.InvalidRequestError: Object '<HostDeployments at
> 0x21cd050>' is already attached to session '1' (this is '3')
>
> Sadly it's not immediately obvious as to what's going on... not even
> certain how
> to start debugging this problem.
>
>
> well an object can only be "attached" to one Session at a time. so the
> tricky thing is that it's kind of easy to get objects in the Session
> sometimes when you don't want them to. like if you have A and B, and
> there's a relationship between both like A.bs an B.a, putting an "A" into a
> Session will pull in the "B" and vice versa.
>
> So if you're putting some kind of object into this other Session to be
> committed immediately, you'd have to make sure it isn't being pulled into
> your primary session. if you call object_session(someobj) on any object
> it will show you what Session it belongs to.
>
> The other option is to merge() the state of the object into another
> Session. merge() makes a copy of an object from one Session to another.
> Though here you're looking to commit those objects in the second session
> and not at all in the first so you probably should jsut make sure those
> objects are only in that one Session.
>
That would explain things, since I have things like:
with isolated_transaction():
host_dep.status = 'ok'
Session.commit()
Obviously the 'host_dep' is from the original session, so this isn't going
to work. I know about merge(),
though in this case I'm not fully certain how to use it, or if it can even
solve issues like this. :(
- Ken
>
>
>
> - Ken
>
>
> On Tue, Jan 8, 2013 at 2:59 PM, Ken Lareau <klar...@tagged.com> wrote:
>
>> On Tue, Jan 8, 2013 at 2:16 PM, Michael Bayer
>> <mike...@zzzcomputing.com>wrote:
>>
>>>
>>> On Jan 8, 2013, at 4:00 PM, Ken Lareau wrote:
>>>
>>>
>>> Given this and your previous comments, and after some conversations with
>>> a coworker, I decided to expose the 'engine' variable from the database
>>> library and create a context manager as so:
>>>
>>> @contextlib.contextmanager
>>> def isolated_transaction(NewSession=None):
>>> """Manage a new transaction (session) within an existing session"""
>>>
>>> needed_session = True
>>> existing = Session.registry()
>>>
>>> if NewSession is None:
>>> NewSession = scoped_session(sessionmaker(bind=engine))
>>> else:
>>> needed_session = False
>>>
>>> Session.registry.set(NewSession)
>>>
>>> try:
>>> yield
>>> finally:
>>> Session.registry.set(existing)
>>>
>>> if needed_session:
>>> NewSession.close()
>>>
>>>
>>> (This code hasn't quite been tested yet and may need some tweaking)
>>> With this, whenever I need an 'isolated' transaction I can simply do
>>> from my program:
>>>
>>> with isolated_transaction():
>>> <do stuff with Session>
>>>
>>> And things should 'just work'... at least that's my hope! I can also
>>> pass
>>> an existing new session if I want to use one in multiple places (not
>>> sure I'll need that just yet, but it was easy enough to write into the
>>> method, so...)
>>>
>>> Hopefully this seems like a sane solution to my problem. :)
>>>
>>>
>>> its good, though you can save yourself all the trouble with
>>> scoped_session/sessionmaker by just saying this:
>>>
>>> from sqlalchemy.orm import Session
>>> new_session = Session(bind=engine)
>>>
>>> or which might be even more portable, use the sessionmaker from your
>>> existing registry:
>>>
>>> new_session = Session.session_factory()
>>>
>>> session_factory is the sessionmaker() you stuck onto scoped_session in
>>> the first place. In that case you don't need to refer to the Engine
>>> explicitly.
>>>
>>
>> Nice, the second one definitely works and I no longer need to export
>> 'engine'
>> and only need to import Session from my library where I have the context
>> manager place. :)
>>
>> Thanks again!
>>
>> - Ken
>>
>>
>>>>> Depending on the database in use, using low isolation levels can have
>>>>> the effect that other transactions can view "dirty reads" as the
>>>>> transaction proceeds, but this is obviously an all-or-nothing thing.
>>>>> When I need certain resources exposed during a long running transaction,
>>>>> I
>>>>> transfer that data to a different Session and commit() those changes
>>>>> distinctly.
>>>>>
>>>>> My current needs would tend to use the short transactions for things
>>>> that are mostly
>>>> isolated from anything going on in the longer running (main)
>>>> transaction, though I do
>>>> suspect I might need what you mention in your last sentence, but might
>>>> you be able
>>>> to refer me to an example of how it would work, perchance?
>>>>
>>>>
>>>> I'd advise against going this route, you'd pretty much need to use
>>>> MySQL MyISAM tables to get guaranteed "dirty reads", that is, there's no
>>>> transaction at all, and it's not really how transactions were meant to be
>>>> used. Lowering the isolation level is usually just a means to get more
>>>> transaction throughput.
>>>>
>>>
>>> Okay, I suspect I misunderstood what was being mentioned here, which is
>>> no problem; I'll avoid it. :) Thanks once again for all the help.
>>>
>>> -Ken
>>>
>>>
>>>>
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>>>>
>>>
>>>
>>>
>>> --
>>> - Ken Lareau
>>>
>>>
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>>
>>
>>
>> --
>> - Ken Lareau
>>
>>
>
>
> --
> - Ken Lareau
>
>
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- Ken Lareau
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