theres no caching in SQLAlchemy. theres an identity map which has
some of the same effects as a cache. basically, if you have already
loaded your object into the current session, thats it. if you load
it again, its going to use the same one you have, unless you clear
the session, or refresh/expire the object. SA is not going to make a
random guess as to what objects you want refreshed and which ones you
are expecting to stay linked to the current editing session, which
changes you want to keep and which ones youd like to discard.
consider this scenario: you load object A and object B into the
current session. objects A and B have state information that is
highly interdependent. you then make changes on B which depend
highly on the current state of A. Then some functions are called to
do some not totally related tasks. As a matter of course, within
those function calls some other part of the application makes a
select statement which also requests "A", which has now been modified
in the database by another process.
returning from the function, A has been magically updated by an
unknown actor, and B is now in an invalid state against the new
version of A. the application breaks.
the point of the identity map is to prevent the above scenario, and
the myriad of variations on it, from occurring. you are assured that
every object, once loaded into the session, will not change
unexpectedly without your say-so, and because their identity is
assured, all changes made to that object identity are reflected
consistently throughout the editing session; its state will always be
consistent within the session. if you need multiple copies/states,
then you use multiple sessions, or pick and choose who should be
refreshed.
if you havent already, I totally overhauled the unit of work
documentation yesterady in response to the recent spate of confusion
over what an identity map and a unit of work is used for - http://
www.sqlalchemy.org/docs/unitofwork.myt . It also details the refresh/
expire/expunge methods as well as "per-object sessions" which may
also provide a way for you to get fresh copies (i.e. myobj =
mapper.using(Session()).get(1) will always be fresh...and unsaveable
unless you import it into another session since the original one
falls out of scope)
also, I would recommend Fowler's "Patterns of Enterprise
Architecture" which is where I learned about all these patterns:
http://www.martinfowler.com/books.html#eaa . SQLAlchemy's ORM is a
python version of the "unit of work", "data mapper", and "identity
map" patterns described in this book. If those arent the patterns
you want to use, its going to be more of an uphill battle trying to
make SA be something its not....but even then, it is possible, youd
just have to write a layer on top of it to provide the patterns youre
looking for (or write an ORM on top of the SQL construction facilities).
On Mar 26, 2006, at 1:33 PM, Florian Boesch wrote:
Seemingly SA is fairly liberal with interpreting my wish to get
stuff from the
database.
So what's the pattern to deal with "session" "caching"?
I hear there's expunge etc. now, and expire, but that's all when I
know what
objects I want to have removed/expired/refreshed. Fact is, I don't,
I wouldn't
ask for an object from the database when I already had that very
object...
So is there a way to end a session by force?
Can I disable all caching?
How do I deal with that?
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