Please read for an updated status on RailsCasts:
Learn more or hide this
GitHub User: hejelin
Maybe not relevant now, but I had the same problem and found the solution in my PostsController under update. I just needed to add :tag_list to the permitted parameters like so:
class PostsController < ApplicationController
if @post.update(params[:post].permit(:title, :text, :tag_list))
Hope it may help someone else in the future.