Re: [sqlalchemy] Query caching allows two objects representing same underlying data in session?
On Mon, Jul 15, 2013 at 8:02 PM, Michael Bayer mike...@zzzcomputing.com wrote: On Jul 15, 2013, at 4:51 PM, Amir Elaguizy aelag...@gmail.com wrote: I'm having this weird problem using the query caching recipes in which two instances of a model representing the same underlying dataset will both get into the session. I know this is happening because I put all of the models in a set() and there are two instances with the same underlying database row id. I was under the impression that the session itself would handle the case that an object coming from the query cache is already in the session, preventing duplication. Is this not the case? well you need to be using the merge() aspect of it, which will reconcile an existing identity that's already in the session. the recipe as written uses merge_result() so will ensure this, yes.This only deals with the identity map though, if you have an object pending with a given identity, its not in the identity map. I'd advise against heavy usage of cached queries overlapping with lots of pending objects within the same sets because things can get very crazy. Because of all that crazyness, that I came to the realization that you can only cache detached instances, and you can never merge them into your session. If you do, you may end up with a concurrency mess, if two threads want to merge the same cached instance into a session. To put a cached instance into a session, you must first copy it, then update. How to do that, is very application-specific, and I don't think it can be automated. -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/groups/opt_out.
Re: [sqlalchemy] Query caching allows two objects representing same underlying data in session?
On Jul 16, 2013, at 1:47 PM, Claudio Freire klaussfre...@gmail.com wrote: On Mon, Jul 15, 2013 at 8:02 PM, Michael Bayer mike...@zzzcomputing.com wrote: On Jul 15, 2013, at 4:51 PM, Amir Elaguizy aelag...@gmail.com wrote: I'm having this weird problem using the query caching recipes in which two instances of a model representing the same underlying dataset will both get into the session. I know this is happening because I put all of the models in a set() and there are two instances with the same underlying database row id. I was under the impression that the session itself would handle the case that an object coming from the query cache is already in the session, preventing duplication. Is this not the case? well you need to be using the merge() aspect of it, which will reconcile an existing identity that's already in the session. the recipe as written uses merge_result() so will ensure this, yes.This only deals with the identity map though, if you have an object pending with a given identity, its not in the identity map. I'd advise against heavy usage of cached queries overlapping with lots of pending objects within the same sets because things can get very crazy. Because of all that crazyness, that I came to the realization that you can only cache detached instances, and you can never merge them into your session. If you do, you may end up with a concurrency mess, if two threads want to merge the same cached instance into a session. a merge() creates a copy of the object. So the model is, the object that is actually *in* the cache remains detached. but you always create an attached copy of it to work with locally. That object can then fire off loaders for additional collections and relationships and such, and if you use the caching recipe fully, those relationships themselves can be cached. It's much more efficient to store a cached version of an A associated via many to one to a B which is many to one to a C as the A, B, C separately, rather than as a graph - otherwise all the A's B's are recaching redundant B's C's, taking up lots more space in the cache and over the network. To put a cached instance into a session, you must first copy it, then update. How to do that, is very application-specific, and I don't think it can be automated. that's what merge(don't_load=True) does. If your object has other aspects to it that are outside of what's mapped, then there are event hooks you can use to re-instantiate additional state. -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/groups/opt_out.
Re: [sqlalchemy] Query caching allows two objects representing same underlying data in session?
On Tue, Jul 16, 2013 at 4:22 PM, Michael Bayer mike...@zzzcomputing.com wrote: To put a cached instance into a session, you must first copy it, then update. How to do that, is very application-specific, and I don't think it can be automated. that's what merge(don't_load=True) does. If your object has other aspects to it that are outside of what's mapped, then there are event hooks you can use to re-instantiate additional state. Oh... didn't know that. That's interesting. I've never toyed with the event system before, but I guess there's an incentive right there. -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/groups/opt_out.
[sqlalchemy] unregister mapper event
Is it possible to remove mapper events? Specifically I want to call event.remove(mapper, 'mapper_configured', fn) but I get an error back saying Mapper is not iterable. -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/groups/opt_out.
Re: [sqlalchemy] unregister mapper event
It's an unimplemented TODO right now.There are ways to remove the events through non-public means, but for it to be done correctly in all cases, there are lots of twists and turns that need to be managed (such as, unregistering an event on an abstract base, an event that has propagate=True, etc).For the full feature add, I will probably have to find some way to have an event function keep track of every listener collection it becomes a part of, so that it can be removed from all of them in a simple way. I'd advise first trying not to rely upon needing to remove events, after that I'd say create your own mutable collection of listeners for those cases where you really need to add/remove them: listeners = set() @event.listens_for(MyClass, my_event) def dispatch(cls, mapper): for fn in listeners: fn(cls, mapper) you can add/remove functions from listeners above. On Jul 16, 2013, at 7:24 PM, Greg Yang sorcerero...@gmail.com wrote: Is it possible to remove mapper events? Specifically I want to call event.remove(mapper, 'mapper_configured', fn) but I get an error back saying Mapper is not iterable. -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/groups/opt_out. -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/groups/opt_out.
