Sounds like you need an atomic operation.
Instead of doing a SELECT, do an UPDATE and then check the affected
rows, such as:
UPDATE table SET flag = 1 WHERE flag = 0;
Then check if the operation succeeded.
Cheers,
Dan
On Sep 30, 11:38 am, gustavo Cardoso <gnasce...@gmail.com> wrote:
> Acctually,
>
> I need to do a SELECT in a table. In this table has a flag field. if the
> flag field has 0 so I can update de flag field to 1.
> In other words I need to do a SELECT by verifing if the flag field is 0. And
> after, I can update to 1 the flag field.
>
> But the problem is: More than one access occurr in this table.
>
> Imagine A and B (two users) accessing this table.
> A do a SELECT.
> B do a SELECT.
> A verify that the flag field is 0.
> B verify that the flag field is 0.
> A UPDATE to 1 the flag field.
> B UPTADE to 1 the flag field. (Wrong!! B couldn't set the flag to 1
> because the flag already was configured to 1.)
>
> But B must to wait for A. A need to put the flag in 0 again. So B can put
> the flag in 1 and use the datas.
>
> is there a way to resolve this problem in django?? Can put some example?
>
> thanks
>
> On Wed, Sep 30, 2009 at 10:05 AM, Kristaps Kūlis
> <kristaps.ku...@gmail.com>wrote:
>
>
>
>
>
> > Create locks and store them at cache as keys.
> > Or implement counter as cache.incr('users_online')
>
> > On Wed, Sep 30, 2009 at 3:52 AM, Nan <ringe...@gmail.com> wrote:
>
> >> Any particular reason you can't just use User.objects.count() ?
>
> >> On Sep 29, 7:16 pm, gustavo Cardoso <gnasce...@gmail.com> wrote:
> >> > Hello Folks,
>
> >> > I need to implement a Counter.
> >> > One variable to store the number of users in my site.
> >> > This variable can't be access in the same time for more than one user.
> >> > How I do this in django. How I create a method that only one user can
> >> access
> >> > in the same time?
> >> > Is there some API to lock and release that variable?
> >> > Thanks!
>
> >> > --
> >> > Gustavo Nascente Cardoso
> >> > Ciência da Computação - UFES
>
> --
> Gustavo Nascente Cardoso
> Ciência da Computação - UFES
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