Rails - Jun 2006 - Sort table by child row property?

I''m building a simple forum system which features a topics and a posts 
table. The topic model has a has_many association with the post model, 
i.e.:

class Topic < ActiveRecord::Base {
  has_many :posts
}

class Post < ActiveRecord:Base {
  belongs_to :topic, :dependent => true
}

Now, I wish to sort the topics by the created_at property of the last 
post, but I can''t for the life of me figure out how.

Any help would be greatly appreciated. I''ve spent the last two days 
trying to solve this.

-- 
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hi there,
there?s a couple of ways to do that in code.. but that would involve a 
bit of sorting through arrays and stuff.
i think the quickest way to accomplish what you want to do is to just 
put a last_post_created_at field into your topic model and just update 
that each time someone writes a new post for that topic...
kind regards,
alexander


Alex Griffioen wrote:
>I''m building a simple forum system which features a topics and a
posts
>table. The topic model has a has_many association with the post model, 
>i.e.:
>
>class Topic < ActiveRecord::Base {
>  has_many :posts
>}
>
>class Post < ActiveRecord:Base {
>  belongs_to :topic, :dependent => true
>}
>
>Now, I wish to sort the topics by the created_at property of the last 
>post, but I can''t for the life of me figure out how.
>
>Any help would be greatly appreciated. I''ve spent the last two days
>trying to solve this.
>
>  
>

alexander wrote:
> i think the quickest way to accomplish what you want to do is to just
> put a last_post_created_at field into your topic model and just update
> that each time someone writes a new post for that topic...
> kind regards,
> alexander
I''ve tried putting a last_post_id in the topics table, and reference it
with:

class Topic < ActiveRecord::Base {
  belongs_to :last_post, :class_name => ''Post'',
:foreign_key =>
''last_post_id''
}

But that caused the posts counter_cache on topic.posts_count to stop 
working and I couldn''t quite find the right place/hook to set the 
last_post association :(

Again, any advice is much appreciated.

-- 
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oh.
i?m sorry.. that?s not what i meant.
my suggestion was something like this:

def whateversavesyourposts
    code that saves your post here
    @topic = Topic.find(@post.topic_id)
    @topic.last_post_created_at(@post.created_at)
    @topic.save
end

i hope this is a bit clearer :)

Alex Griffioen wrote:
>alexander wrote:
>  
>
>>i think the quickest way to accomplish what you want to do is to just
>>put a last_post_created_at field into your topic model and just update
>>that each time someone writes a new post for that topic...
>>kind regards,
>>alexander
>>    
>>
>
>I''ve tried putting a last_post_id in the topics table, and
reference it
>with:
>
>class Topic < ActiveRecord::Base {
>  belongs_to :last_post, :class_name => ''Post'',
:foreign_key =>
>''last_post_id''
>}
>
>But that caused the posts counter_cache on topic.posts_count to stop 
>working and I couldn''t quite find the right place/hook to set the 
>last_post association :(
>
>Again, any advice is much appreciated.
>
>  
>

alexander wrote:
> oh.
> i?m sorry.. that?s not what i meant.
> my suggestion was something like this:
> 
> def whateversavesyourposts
>     code that saves your post here
>     @topic = Topic.find(@post.topic_id)
>     @topic.last_post_created_at(@post.created_at)
>     @topic.save
> end
> 
> i hope this is a bit clearer :)
I know ;) But I thought: "Why save just the date if I could reference 
the entire post?" I know RForum, for instance, saves both the 
last_post_date and last_post_author_id... that seemed a bit nonsensical 
if you can just save the last post :)

-- 
Posted via http://www.ruby-forum.com/.

Alex Griffioen <info@...> writes:
> I know ;) But I thought: "Why save just the date if I could reference 
> the entire post?" I know RForum, for instance, saves both the 
> last_post_date and last_post_author_id... that seemed a bit nonsensical 
> if you can just save the last post :)
It''s all about saving a join with the post tables. The query used to
fetch the
article "headers" doesn''t touch the posts table at all.

-- Pazu

Pazu wrote:
> It''s all about saving a join with the post tables. The query used
to
> fetch the
> article "headers" doesn''t touch the posts table at all.
> 
> -- Pazu
You''re saying it''s smart to store last_post_created_at and 
last_post_author_id in the topics table, right? I still don''t see
why...
I''d rather use eager loading to fetch the topic''s last post,
but I can''t
seem to assign topic.last_post properly :(

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