>> 1) retrieve the current_high_bid field of the Auction object
>> 2) compare it with the new_bid
>> 3) if the new bid is higher than current_high_bid, I update
>> current_high_bid to the value of the new_bid.
>>
>> but there's a (slight) chance that between steps (1) and (3) a
>> different user could've submitted an even higher bid which then become
>> the current_high_bid. In that case step 3 should not proceed. What can
>> I do to make sure such a situation does not arise?
>
> For now, you can write custom SQL (consider this pseudo-code rather
> than a drop-in replacement):
>
> """ Be sure this all runs inside a single transaction! Use the
> appropriate middleware or
> decorators... """
> cursor = connection.cursor()
> cursor.execute("SELECT high_bid FROM auctionapp_auction WHERE id=%s
> FOR UPDATE", [auction.id])
> """ Until this SELECT, other users could have modified the high
> bid, so do not trust the
> in-memory copy. """
> returned_rows = cursor.fetchall()
> high_bid = returned_rows[0][0]
> if new_bid > high_bid:
> auction.high_bid = new_bid
> auction.save()
If you are going to drop to raw SQL, you can let SQL handle the
atomicity:
UPDATE auction SET
current_high_bid = %(new_bid)f
WHERE id = %(auction_id)i
AND %(new_bid)f > current_high_bid
If the new_bid is based on data in another table of
bid-histories, you can use something like
UPDATE auction SET
current_high_bid = (
SELECT Max(bid)
FROM Bids
WHERE auction_id = %(auction_id)s
)
WHERE id = %(auction_id)i
AND (
SELECT Max(bid)
FROM Bids
WHERE auction_id = %(auction_id)s
) > current_high_bid
Adjust for table-names and field-names. It should be fairly
portable SQL, working across all of the big 3 Django databases
(sqlite, postgres, and recent versions of mysql that support
sub-selects...it also happens to work in SQL Server, FWIW)
-tim
--~--~---------~--~----~------------~-------~--~----~
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
-~----------~----~----~----~------~----~------~--~---
