ordering by child's value in one to many relationship

posts table

int id

comments table

int id
int post_id
datetime created_at

i need to orde开发者_如何学运维r posts by post's comments created_at. I tried something like that but i can't select distinct post ids.Any help will be appreciated.

Post.all(:joins => :comments, :order => 'comments.created_at')    

I want to see the post which was commented lately at the top.

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The condition you are passing is invalid. The easiest way you can accomplish what you are trying to do, is by adding a column to your posts table - say last_commented_at. In your Comment model you add a callback

class Comment < ActiveRecord::Base
  belongs_to :post

  after_save :update_posts_last_commented_attribute

  def update_posts_last_commented_attribute
    post.update_attribute(:last_commented_at, updated_at)
  end
end

Then you can load your Posts by calling

Post.order("last_commented_at DESC" )

to show the posts first, that have recently been commented.