> But, I guess you can add a method (or override the save() method maybe), to > send the correct SQL query.
Writing my own SQL for each time I'd like to maintain data integrity is something I'm not really interested in. I'm using Django to avoid writing SQL. Also, the cases are much more complicated than val+=1. They are multiple lines of python operations on models. I just used that as an example to illustrate the problem. A lock is the real solution, but I have no idea how to implement it properly. Ivan On Jan 22, 1:07 pm, Jonathan Ballet <[EMAIL PROTECTED]> wrote: > Hi, > > Le Tue, 22 Jan 2008 11:07:45 -0800 (PST), "[EMAIL PROTECTED]" > <[EMAIL PROTECTED]> a écrit : > > > Example: > > Processes P1, P2, model instance m, with m.val > > > - P1 grabs the object : m = MyModel.objects.filter(get the m i want) > > [0] > > - P2 grabs the object : m = MyModel.objects.filter(get the m i want) > > [0] > > - P1 m.val += 1 > > - P2 m.val += 1 > > - P1 m.save() > > - P2 m.save() > > > What happens normally? What happens with the transaction middleware? > > I guess the save() methods will set directly the new value, instead of > sending a "val + 1" UPDATE to the database. > > So, if m.val == 0 when you load the object in both P1 and P2, it will > result in : > > UPDATE mymodel SET val = 1 WHERE id = ? > > whereas you want : > > UPDATE mymodel SET val = val + 1 WHERE id = ? > > In the latter, the transaction will handle this (well, as Jeff Anderson > said, the _database_ will handle this), and your "val" field will be +2. > > However, I'm pretty sure Django will do the former. It needs to be > checked. > But, I guess you can add a method (or override the save() method > maybe), to send the correct SQL query. > > - Jonathan --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/django-users?hl=en -~----------~----~----~----~------~----~------~--~---
