[sqlalchemy] Re: Events Undo/Redo Functionality?
these are two general patterns, observer/listener and undo/redo (command-pattern), which have nothing to do with DB. i dont know about 2) DB-observers which would react on somebody else changing the DB, AFAIK that is possible on certain servers only, but the rest should be done independly on higher level than DB-stuff. read about Model-View-Controller. On Tuesday 24 February 2009 15:00:28 paniq303 wrote: Hi everyone, I am developing an offline music synthesizing and sequencing application in Python, using the SQLite backend to store song data. At the moment, I am using my own object relational mapper between SQL and Python, which is fairly minimal. I feel compelled to use SQLAlchemy, but there are two areas where I would like to assess my options before I go forward and embed SQLAlchemy into my application. 1) Is it possible/planned to connect signal handlers to database changes? In my app, changing object properties will call connected event handlers, which in turn refresh UI views. How could I do this with SQLA? Can you think of another way to update the UI from DB changes? Another way could be to have a hook listen to transactions and generate journal records from which events can be signalled - this would even enable multiple processes to handle events. Does that sound more plausible? Is it possible? 2) I see that SQLA supports rollback for pending transactions, which is a great feature. But as far as I understand, it is not possible to rollback transactions that have already been committed (undo). How would I go about and implement such a feature into my application best, using SQLA as a backend? I understand that SQL is traditionally used in web services, where event handling is virtually impossible and extensive undo/redo functionality is rarely needed. But I believe it makes perfect sense to use databases for document formats as well. Looking forward to your answer. --~--~-~--~~~---~--~~ You received this message because you are subscribed to the Google Groups sqlalchemy group. To post to this group, send email to sqlalchemy@googlegroups.com To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en -~--~~~~--~~--~--~---
[sqlalchemy] Re: Events Undo/Redo Functionality?
On Feb 24, 5:16 pm, a...@svilendobrev.com wrote: these are two general patterns, observer/listener and undo/redo (command-pattern), which have nothing to do with DB. i dont know about 2) DB-observers which would react on somebody else changing the DB, AFAIK that is possible on certain servers only, but the rest should be done independly on higher level than DB-stuff. read about Model-View-Controller. Your reply sounds a bit offended. I'm sorry if my request came over as kind of rude. I don't mean to upset anybody, and I don't want to take up your time, so please send me elsewhere if my inquiry does not fit the projects scope. I'm using a Model-View-Controller scheme at the moment. I also know of the command pattern and how it works. But I'm trying to write as little code on my own as I can. SQLAlchemy provides a high level model part for the application, and borders on the issues I presented above. I can see that the issues I presented are not neccessarily of importance in the scope of SQL. So let me rephrase my question: how could I solve above two issues in the most efficient manner, leveraging whatever SQL and SQLAlchemy have to offer? Is there any short cut I can take on the way to undo/redo functionality? If journaling DB changes would be a good way to track changes, how can I hook up on them best? Where would my plan fit in? I'm sorry if these questions swamp you. Maybe I am thinking too much. --~--~-~--~~~---~--~~ You received this message because you are subscribed to the Google Groups sqlalchemy group. To post to this group, send email to sqlalchemy@googlegroups.com To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en -~--~~~~--~~--~--~---
[sqlalchemy] Re: Events Undo/Redo Functionality?
If you choose to go down the journaling route, some things would be helpful: 1. all of your functions (journalable actions) should have inverse functions to reverse them. If this is true, then from a particular state, you can simply record the actions, checkpointing all user objects periodically. A sequence might be something like: save binary state of file cut(1:30, 1:40) loop(a,13, sec=10) Since you described an music editing app, doing this in clean way (without losing precision and information) might require storing actual binary states. This sounds, to me, a little nightmarish. I don't have any insights on the SA front, alas. Gregg Lind On Tue, Feb 24, 2009 at 2:41 PM, paniq303 paniq...@googlemail.com wrote: On Feb 24, 5:16 pm, a...@svilendobrev.com wrote: these are two general patterns, observer/listener and undo/redo (command-pattern), which have nothing to do with DB. i dont know about 2) DB-observers which would react on somebody else changing the DB, AFAIK that is possible on certain servers only, but the rest should be done independly on higher level than DB-stuff. read about Model-View-Controller. Your reply sounds a bit offended. I'm sorry if my request came over as kind of rude. I don't mean to upset anybody, and I don't want to take up your time, so please send me elsewhere if my inquiry does not fit the projects scope. I'm using a Model-View-Controller scheme at the moment. I also know of the command pattern and how it works. But I'm trying to write as little code on my own as I can. SQLAlchemy provides a high level model part for the application, and borders on the issues I presented above. I can see that the issues I presented are not neccessarily of importance in the scope of SQL. So let me rephrase my question: how could I solve above two issues in the most efficient manner, leveraging whatever SQL and SQLAlchemy have to offer? Is there any short cut I can take on the way to undo/redo functionality? If journaling DB changes would be a good way to track changes, how can I hook up on them best? Where would my plan fit in? I'm sorry if these questions swamp you. Maybe I am thinking too much. --~--~-~--~~~---~--~~ You received this message because you are subscribed to the Google Groups sqlalchemy group. To post to this group, send email to sqlalchemy@googlegroups.com To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en -~--~~~~--~~--~--~---
