Would something like
UPDATE bid = new_bid WHERE id = id and bid < new_bid
work for you?
It is a more optimistic approach (it assumes that the case you describe is
an exception rather than what usually happens) and I think it is simpler by
not having to do any locking etc.
Regards,
Tamas
On Tue, Nov 10, 2009 at 2:17 PM, Continuation <selforgani...@gmail.com>wrote:
>
> Thanks Christophe and Kenneth!
>
> Let me make sure I understand this:
>
> If I write this vew function:
>
> @transaction.commit_on_success
> def update_high_bid(request):
> ....
> cursor = connection.cursor()
> cursor.execute("SELECT high_bid FROM auctionapp_auction WHERE
> id=%s
> FOR UPDATE", [auction.id])
> returned_rows = cursor.fetchall()
> high_bid = returned_rows[0][0]
> if new_bid > high_bid:
> auction.high_bid = new_bid
> auction.save()
>
> The entire function will be wrapped within a transaction.
> SELECT FOR UPDATE will acquire a row-level lock
> and that lock will not be released until the function update_high_bid
> () returns successfully, or until the function raises an exception and
> the whole transaction is rolled back.
>
> Is that right?
>
> Thanks.
>
>
> On Nov 10, 12:45 am, Christophe Pettus <x...@thebuild.com> wrote:
> > On Nov 9, 2009, at 9:34 PM, Continuation wrote:
> >
> > > Also does django middleware acquire database lock on my behalf, or do
> > > I need to explicitly perform the locking?
> >
> > In the example code, it's the SELECT ... FOR UPDATE that acquires the
> > lock. Django doesn't currently have any explicit knowledge of
> > locking, so you need to drop down to the custom SQL level to issue the
> > right statement to acquire the lock on the row.
> >
> > The example I wrote assumed you were using PostgreSQL as the backend;
> > you can get all sorts of details about locking in PostgreSQL here:
> >
> >
> http://www.postgresql.org/docs/8.4/interactive/explicit-locking.html#...
> > --
> > -- Christophe Pettus
> > x...@thebuild.com
> >
>
--~--~---------~--~----~------------~-------~--~----~
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
django-users+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/django-users?hl=en
-~----------~----~----~----~------~----~------~--~---
