Rails - Jun 2007 - How to sort on a join... kindof.

So i''ve got two models:

foo has_many bars
bar belongs_to foo

And then I can find the newest bar since I''m using nifty created_at
columns...

@foo=Foo.find(1)
@foo.bar.last.whatever

But lets say I have much foo

@foos=Foo.find(:all)
@foos.each do | f |
f.bar.last.whatever
end

But what I really want to do is sort the foos in order of the last bar
created for each foo.  But I can''t figure out how to do it.  The
specifics are topics and posts, and of course I''m trying to do a
paginating find...

    @topics_pages, @topics = paginate(:topics, :conditions =>
["group_id = ?", @group.id], :order => "what in the world do i
put
here...")

It''s almost like I have to do a join in sql here, but then I end up
pulling in all the posts in to the array at the time of the find, when
all I want to do is get the topic so I can later do a topic.post.last

Any great ideas?


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OK, so I think I can just get the topics into @topics, and then sort
the array in rails on topic.post.last, but I don''t know how to do
it... am I going down the right path here?

---A

On Jun 21, 9:45 pm,
"andrew.ohns...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org"
<andrew.ohns...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> So i''ve got two models:
>
> foo has_many bars
> bar belongs_to foo
>
> And then I can find the newest bar since I''m using nifty
created_at
> columns...
>
> @foo=Foo.find(1)
> @foo.bar.last.whatever
>
> But lets say I have much foo
>
> @foos=Foo.find(:all)
> @foos.each do | f |
> f.bar.last.whatever
> end
>
> But what I really want to do is sort the foos in order of the last bar
> created for each foo.  But I can''t figure out how to do it.  The
> specifics are topics and posts, and of course I''m trying to do a
> paginating find...
>
>     @topics_pages, @topics = paginate(:topics, :conditions =>
> ["group_id = ?", @group.id], :order => "what in the world
do i put
> here...")
>
> It''s almost like I have to do a join in sql here, but then I end
up
> pulling in all the posts in to the array at the time of the find, when
> all I want to do is get the topic so I can later do a topic.post.last
>
> Any great ideas?

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:order => "created_at"
or
:order => "created_at DESC"

based on which way u want it sorted.

If you wanted to sort a collection, you could also do:

---------
for thing in @things.sort { b.created_at <=> a.created_at }
 #loop thru your things and display them
end
---------

or you could use sort_by (see link below).

and for more on sorting:
http://redcorundum.blogspot.com/2006/10/using-sortby-instead-of-sort.html


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Dave Coleman wrote:
> ---------
> for thing in @things.sort { b.created_at <=> a.created_at }
>  #loop thru your things and display them
> end
> ---------
I probably should have read what I typed. it would be:

---------
for thing in @things.sort {|a,b| b.created_at <=> a.created_at }
 #loop thru your things and display them
end
---------

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On Fri, 22 Jun 2007, Dave Coleman wrote:
> 
> ---------
> for thing in @things.sort {|a,b| b.created_at <=> a.created_at }
>  #loop thru your things and display them
> end
> ---------
More idiomatically:
  @things.sort_by {|x| x.created_at}.each do |thing|
    #loop through your things and display them
  end

Ought to be quicker, but will use more memory.

----------------------------------------------------- Enumerable#sort_by
     enum.sort_by {| obj | block }    => array
------------------------------------------------------------------------
     Sorts _enum_ using a set of keys generated by mapping the values in
     _enum_ through the given block.

        %w{ apple pear fig }.sort_by {|word| word.length}
                     #=> ["fig", "pear",
"apple"]
        [...]

        Hugh

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I''d rather do it in SQL, but I didn''t think :order =>
"created_at"
would work... the data being pulled in is a list of topics.  I do not
want to sort on the "created_at" of the topic, but on the
"created_at"
of the last post associated with the topic.

Topic 1 was created 2 days ago.  It has 2 posts, the newest post is 4
hours old.
Topic 2 was created 1 day ago.  It has 2 posts, the newest post is 8
hours old.
Topic 3 was created 1 hour ago.  It has 2 posts, the newest post is 30
minutes old.

The correct sort order should be 3, 1, 2.  If I just do :order =>
"created_at" then the order will be 3, 2, 1.

I am using the .last function to find the last post, but of course
that only exists in rails.  If I specify the post<->topic join in
the .find then I can do :order => "posts.created_at" but the side
effect of that is a huge array returned to rails with all the posts
for a particular topic, when all I need is the created_at of the .last
one.

I think I''m looking at a sort in rails, after the list of topics has
been pulled from the db.  Rails'' .sort and sort_by do not make any
sense to me yet, so I''ll have to do some reading and figure them
out.  :)


---A

On Jun 22, 11:38 am, Dave Coleman
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
> :order => "created_at"
> or
> :order => "created_at DESC"
>
> based on which way u want it sorted.
>
> If you wanted to sort a collection, you could also do:
>
> ---------
> for thing in @things.sort { b.created_at <=> a.created_at }
>  #loop thru your things and display them
> end
> ---------
>
> or you could use sort_by (see link below).
>
> and for more on
sorting:http://redcorundum.blogspot.com/2006/10/using-sortby-instead-of-sort....
>
> --
> Posted viahttp://www.ruby-forum.com/.

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