Question

Im using the will_paginate gem and rails 3.2.3.
Im trying to display all the posts that have certain tags ordered by the latest post so I use this:

  @posts = Post.find_by_post_type_label(session[:current_post_type],@selected_labels)
             .paginate(:page => params[:page], :order => "comments.created_at DESC").all

but it returns the posts out of order. Each page will be sorted, but the second page will have posts that should have been on the first one.

Example:

Post.find_by_post_type_label(1,[])

returns posts [1,2,3,4,5,6,7,8,9,10] with each ones latest comment being that many months ago. The sql it generates is:

 SELECT `posts`.* FROM `posts` WHERE `post`.`post_type_id` = 1

and

 SELECT `comments`.* FROM `comments` WHERE `comments`.`post_id` IN ( 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ) ORDER BY created_at DESC

If I order it, and limit it though, like:

Post.find_by_post_type_label(1,[]).order(comments.created_at DESC).limit(5)

which is how the will_paginate does it, it returns posts 2,3,4,5,7

after generating this:

SELECT DISTINCT `post`.id FROM `posts` LEFT OUTER JOIN
`comments` ON `comments`.`post_id` = `post`.`id` WHERE 
`post`.`post_type_id` = 1 ORDER BY posts.created_at DESC LIMIT 5

and

 SELECT `post`.`id` AS t0_r0, `post`.`user_id` AS t0_r1, `post`.`name` AS 
 t0_r2, `post`.`image_id` AS t0_r3, `post`.`post_type_id` AS t0_r4, 
`post`.`description` AS t0_r5, `post`.`featured` AS t0_r6, `post`.`approved`
AS t0_r7, `comment`.`id` AS t1_r0, `comments`.`post_id` AS t1_r1, 
`comments`.`image_id` AS t1_r2, `comments`.`edits` AS t1_r3, 
`comments`.`color` AS t1_r4, `comments`.`min_reputation` AS t1_r5, 
`comments`.`created_at` AS t1_r6, `comments`.`updated_at` AS t1_r7 FROM 
`post` LEFT OUTER JOIN `comments` ON `comments`.`post_id` = 
`post`.`id` WHERE `post`.`post_type_id` = 1 AND `post`.`id` IN (2, 3, 
4, 5, 7) ORDER BY comments.created_at DESC

It is clearly limiting before it is ordering, but I cant figure out how to change that.
Any help would be appreciated

Edit 1

The relationships are like so:

Class Post
  belongs_to :user
  belongs_to :post_type
  has_many :comments, dependent: :destroy, order: 'created_at DESC'
  has_and_belongs_to_many :labels

Class Comment
  belongs_to :post
  belongs_to :user

Comments are the only ones with timestamps in the database. The first comment holds the created at time for the post.

Answer 1

Winded up using this sql in a find_by_sql

SELECT  p1.*, iv.created_at FROM posts p1
INNER JOIN ( SELECT p2.id id, max(c.created_at) created_at FROM posts p2
            INNER JOIN comments c ON c.post_id = p2.id
            GROUP BY p2.id)
iv ON p1.id=iv.id ORDER BY iv.created_at DESC LIMIT 5