From: Phlip <phlip2005@...>
Date: 2009-02-23T11:59:29+09:00
Subject: Re: Using Find based on method rather than table

Jon Hope wrote:

> Topic.find(:all, :order => 'lastest_post_date DESC')
> 
> it throws an error, as it looks for a table rather than using the method
> I've written.
> 
> There must be an easy way to do this in Ruby!

Nope!

Firstly, this is an ActiveRecord question, and you should always post to the 
narrowest technical newsgroup for the best answer. Although this particular 
question is easy, don't take my follow-up as encouragement to abuse this group! 
Try the Ruby on Rails Talk forum at Google Group for general AR and RoR questions...

In general, find arguments, like :order's target, must appear in your database. 
(As a field, not a "table"). That's because .find's job is to construct a wicked 
long SQL SELECT string, and submit this to your database engine - sqlite3, 
MySQL, etc.

They know nothing about Ruby, so they can't just reach back into your program 
and find your latest_post_date.

You could put latest_post_date into the database as a field. Do this if you need 
real speed (unlikely in a young project), or if you need the features discussed 
below.

Next, you could replicate the logic from 'def latest_post_date', by rebuilding 
it in a raw SQL fragment. This has odds of working:

   Topic.find(:all, :include => :posts,
     :order => '(CASE WHEN topics.created_at > posts.created_at
                  THEN topics.created_at ELSE posts.created_at END) DESC')

Crack an SQL tutorial (and _don't_ pester a Ruby or Rails forum about it!) if 
that does not work; I wrote it without testing it or nothing. I also only made a 
guess about the contents of your method.

And you might find it easier to simply sort in Ruby, instead of SQL:

   Topic.all.sort_by(&:latest_post_date)

That _will_ reach out to your method! However...

One joy of SQL and AR is you can use the :page, :offset, and/or :limit fields to 
create a "sliding window" of results. Sometimes a query might return too many 
results for one HTML window, so you should give the user those familiar VCR 
controls to slide forward and back thru a simulated "database cursor".

And that sliding window obviously will not work if you :limit and :offset your 
database query, and only _then_ sort it. The VCR controls will not exactly 
appear to show batches of records in the correct order!

Finally, if you indeed need to use the greater of topics.created_at and 
posts.created_at, you could write a Post.after_save that "bumps" the 
topics.created_at field. Then you don't even need latest_post_date.

-- 
   Phlip
   http://zeekland.zeroplayer.com/Pigleg_Too/1