Rails - Jul 2007 - Sorting By Last Comment? ("post.comments.first.created_on")

I''m trying to order a table based on the last comment, and have tried
almost everything and still can''t get it working.

My specs are just like the API''s
(http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#M000644)

<code>

  class Post < ActiveRecord::Base
    belongs_to :author
    has_many   :comments
  end

Consider the following loop using the class above:

  for post in Post.find(:all)
    puts "Post:            " + post.title
    puts "Written by:      " + post.author.name
    puts "Last comment on: " + post.comments.first.created_on
  end

</code>

I tried doing:

":order => ''@posts.comments.first.created_on
DESC''" -- but can''t get it
working.

How could I order the table based on "post.comments.first.created_on"?

-- 
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Aah,
it appears you misunderstand how the :order key works in a find,
it''s all a question of SQL.

Firstly,
you want to make your code efficient,
so you want to achieve your outcome in as few DB queries as possible.

For every has_many, or belongs_to, it needs to make a database query,
so instead, we''ll "eager" load these when we do our find.

all_with_attributes = Post.find(:all, :include => [:author, :comments]

but, actually, we don''t need this,
if you''re not using the comments, and only the time of the comment,
instead we''ll define a "last_comment" in your class

class Post < ActiveRecord::Base
    belongs_to :author
    has_many   :comments
    has_one    :last_comment,
        :class_name => "Comment",
        :order      => "comments.created_at DESC"
end

This now let''s your write @post.last_comment.created_at

so, now our find should be;

Post.find(:all,
    :include => [:author, :last_comment],
    :order => "comments.created_at DESC"
    )

And I think we''ll have what you need.


If that''s too confusing for you,
you can just do the following in ruby (rather than in the db)

Post.find(:all,
    :include => [:author, :comments]
    ).sort_by{|p| p.comments.last.created_at}

* the include is optional, but seriously, you need to be doing this
* watch your development.log when you don''t

or

Post.find(:all,
    :include => [:author, :comments],
    :order => "comments.created_at DESC"
    )

would probably work too.

Bob Sanders wrote:
> I''m trying to order a table based on the last comment, and have
tried
> almost everything and still can''t get it working.
> 
> 
>   class Post < ActiveRecord::Base
>     belongs_to :author
>     has_many   :comments
>   end
> 
> Consider the following loop using the class above:
> 
>   for post in Post.find(:all)
>     puts "Post:            " + post.title
>     puts "Written by:      " + post.author.name
>     puts "Last comment on: " + post.comments.first.created_on
>   end
-- 
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Matthew! You''re awesome! Thank you so much for your informative post. I
can''t tell you how appreciative I am. Thank you, thank you, thank you
:)

I tried the first method, and it works great. I had issue with 
displaying the latest date, so I used your second approach, 
specifically:

Post.find(:all,
    :include => [:author, :comments],
    :order => "comments.created_at DESC"
    )

Then in the view, I incorporated this: post.comments.first.created_at, 
and it works perfectly like how I wanted.

Thank you so much for your help, Matthew!!


Matthew Rudy wrote:
> Aah,
> it appears you misunderstand how the :order key works in a find,
> it''s all a question of SQL.
> 
> Firstly,
> you want to make your code efficient,
> so you want to achieve your outcome in as few DB queries as possible.
> 
> For every has_many, or belongs_to, it needs to make a database query,
> so instead, we''ll "eager" load these when we do our
find.
> 
> all_with_attributes = Post.find(:all, :include => [:author, :comments]
> 
> but, actually, we don''t need this,
> if you''re not using the comments, and only the time of the
comment,
> instead we''ll define a "last_comment" in your class
> 
> class Post < ActiveRecord::Base
>     belongs_to :author
>     has_many   :comments
>     has_one    :last_comment,
>         :class_name => "Comment",
>         :order      => "comments.created_at DESC"
> end
> 
> This now let''s your write @post.last_comment.created_at
> 
> so, now our find should be;
> 
> Post.find(:all,
>     :include => [:author, :last_comment],
>     :order => "comments.created_at DESC"
>     )
> 
> And I think we''ll have what you need.
> 
> 
> If that''s too confusing for you,
> you can just do the following in ruby (rather than in the db)
> 
> Post.find(:all,
>     :include => [:author, :comments]
>     ).sort_by{|p| p.comments.last.created_at}
> 
> * the include is optional, but seriously, you need to be doing this
> * watch your development.log when you don''t
> 
> or
> 
> Post.find(:all,
>     :include => [:author, :comments],
>     :order => "comments.created_at DESC"
>     )
> 
> would probably work too.
> 
> Bob Sanders wrote:
>> I''m trying to order a table based on the last comment, and
have tried
>> almost everything and still can''t get it working.
>> 
>> 
>>   class Post < ActiveRecord::Base
>>     belongs_to :author
>>     has_many   :comments
>>   end
>> 
>> Consider the following loop using the class above:
>> 
>>   for post in Post.find(:all)
>>     puts "Post:            " + post.title
>>     puts "Written by:      " + post.author.name
>>     puts "Last comment on: " + post.comments.first.created_on
>>   end

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Why are you doing comments.first? You''ll only ever get the most recent
comment if there''s only one there.

post.comments.find(:first, :order => "created_on DESC").created_on

should get you what you need.

Also, just so you know, the Ruby Way of your code is as follows:

Post.find(:all).each do |post|
  ...
end

Jason

On 7/15/07, Bob Sanders
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
>
>
> I''m trying to order a table based on the last comment, and have
tried
> almost everything and still can''t get it working.
>
> My specs are just like the API''s
> (
>
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#M000644
> )
>
> <code>
>
>   class Post < ActiveRecord::Base
>     belongs_to :author
>     has_many   :comments
>   end
>
> Consider the following loop using the class above:
>
>   for post in Post.find(:all)
>     puts "Post:            " + post.title
>     puts "Written by:      " + post.author.name
>     puts "Last comment on: " + post.comments.first.created_on
>   end
>
> </code>
>
> I tried doing:
>
> ":order => ''@posts.comments.first.created_on
DESC''" -- but can''t get it
> working.
>
> How could I order the table based on
"post.comments.first.created_on"?
>
> --
> Posted via http://www.ruby-forum.com/.
>
> >
>
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