Rails - Jun 2009 - counter_cache is making a redundant SELECT before UPDATE

Hi,

I have the following code:

Message, belongs_to :topic, :counter_cache => true

topic = Topic.find_from_permalink(params[:id])

topic.messages.create(params[:message])

When the message gets created, then AR issues a supplemental SELECT to
retrieve the message''s topic and then updates its messages_count. Why
is
that happening?

If I do it manually:

if topic.messages.create(params[:message])
  topic.messages_count += 1
  topic.save!
end

Then I don''t get this additional SELECT. Does the sql cache only work
when I do Topic.find(2) ?
-- 
Posted via http://www.ruby-forum.com/.

Hmm, actually I just watched the railscasts about the counter_cache, and 
in his log he also has this redundant SELECT that he didn''t even spot.

So I guess this could be an additional feature added to Rails to save 1 
DB query.
-- 
Posted via http://www.ruby-forum.com/.

On Jun 1, 10:48 am, Fernando Perez
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
> Hmm, actually I just watched the railscasts about the counter_cache, and
> in his log he also has this redundant SELECT that he didn''t even
spot.
>
> So I guess this could be an additional feature added to Rails to save 1
> DB query.
This is basically rails not knowing about inverse associations (which
it will do in the next version).
If you do do itself you should use increment_counter rather that doing
what you show above (which has a race condition).

Fred
> --
> Posted viahttp://www.ruby-forum.com/.

> This is basically rails not knowing about inverse associations (which
> it will do in the next version).
> If you do do itself you should use increment_counter rather that doing
> what you show above (which has a race condition).
> 
> Fred
Thank you Frederick for your message :-)
-- 
Posted via http://www.ruby-forum.com/.

Quoting Fernando Perez
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>:
> 
> Hmm, actually I just watched the railscasts about the counter_cache, and 
> in his log he also has this redundant SELECT that he didn''t even
spot.
> 
> So I guess this could be an additional feature added to Rails to save 1 
> DB query.
Is the title declared unique?  Then Rails queries the DB to check that there
is no existing record before doing the UPDATE.

HTH,
  Jeffrey