sudo.rb

course = Course.find('biology')

#get post ids, stored as set
post_key_arr = Course.fetch('post_ids', course_id)

#warm cache
if post_arr
  post_key_arr.each do |p|
    post_arr << Post.fetch('id', p.id)
  end
else
  #cold cache
  posts = Post.find_by_course_id(course_id).group(&:post_id)

  post_key_arr = posts.keys
  post_arr = posts.values

  Redis.store_set('post-by-course-' + course_id, post_arr)

  post_arr.each do |p|
    Redis.add('post-' + p.id)
  end
end