comment.rb

class Comment < ActiveRecord::Base
  belongs_to :post
end

comments_controller.rb

class CommentsController < ApplicationController::Base
  def index
    @post = Post.find(params[:post_id])
    if @post
      @search = @post.comments.search(params[:search])
    else
      @search = Comment.search(params[:search])
    end
    @comments = @search.all
  end
end

index.html.erb

<table>
  <tr>
    <%= order @search, :by => :title, :as => 'Title' %>
  </tr>

... 

</table>

post.rb

class Post < ActiveRecord::Base
  has_many :comments
end

routes.rb

ActionController::Routing::Routes.draw do |map|

  map.resources :comments # for overview of all comments
  map.resources :post, :has_many => :comments

end