post.rb

class Post < ActiveRecord::Base

end

posts_controller.rb

class PostsController < ApplicationController

  SECTIONS = %w{seo ruby php java}

  def index
    if params[:latest] == true
      @posts = Post.where("updated_at between now() - interval '7 day' and now()").order("updated_at desc")
    elsif !params[:section].nil? && SECTIONS.include?(params[:section])
      @posts = Post.where("section = #{params[:section]}").order("updated_at desc")
    else
      @posts = Post.order("updated_at desc")
    end

    @comment_sizes = { }
    @posts.each do |post|
      @comment_sizes[post.id] = post.comments.size
    end
  end

  def comments
    if params[:latest] == true
      @posts = Post.where("updated_at between now() - interval '7 day' and now()").order("updated_at desc")
    elsif !params[:section].nil? && SECTIONS.include?(params[:section])
      @posts = Post.where("section = #{params[:section]}").order("updated_at desc")
    else
      @posts = Post.order("updated_at desc")
    end

    @comments = []
    @posts.each do |post|
      @comments += post.comments
    end
  end

end

routes.rb

Simple::Application.routes.draw do
  resources :posts do
    collection do
      get 'comments'
    end
  end
end