Query for results ordered by column of association table, most recent association only

Question

given the following schema

class User
  has_many :page_views
  #[id]
end

class Page
  has_many :page_views
  #[id]
end

class PageView
  belongs_to :user
  belongs_to :page
  #[id, created_at, updated_at]
end

How can i query for a given page, all users most recent page views in order by created_at date. One row per user, showing only their most recent view.

Answer 1

You want the PageViews with the maximum created_at for each user_id for a particular page_id. This is a representation of this query in SQL:

SELECT max(created_at) AS last_visit, page_views.* 
FROM page_views 
WHERE (page_id = 1) 
GROUP BY user_id 
ORDER BY last_visit DESC

In Rails 3, you can write this like so:

Page.first.page_views.group(:user_id).select('max(created_at) AS last_visit, page_views.*').order('last_visit DESC')

Answer 2

Try this:

class Page
  has_many :page_views
  has_many :recent_page_views, :class_name => "PageView",
    :joins => "INNER JOIN (
      SELECT     a.id, COUNT(*) AS ranknum
      FROM       page_views AS a
      INNER JOIN page_views AS b ON a.user_id = b.user_id AND 
                                    a.page_id = b.page_id AND 
                                    a.created_at <= b.created_at
      GROUP BY   a.id
      HAVING COUNT(*) = 1
    ) AS d ON (page_views.id = d.id)",
    :order => "page_views.created_at"

 has_many :recent_viewers, :through => :recent_page_views, 
            :source => :user, :order => "page_views.created_at"
end

Now

page.recent_viewers