Added rest of solution, for parts 4 and 5.

app/controllers/movies_controller.rb

@@ -7,14 +7,22 @@ def show
   end
 
   def index
-    if params[:title_sort]
+    sort = params[:sort] || session[:sort]
+    case sort
+    when 'title'
       ordering,@title_header = {:order => :title}, 'hilite'
-    elsif params[:release_date_sort]
+    when 'release_date'
       ordering,@date_header = {:order => :release_date}, 'hilite'
     end
     @all_ratings = Movie.all_ratings
-    @selected_ratings = (params[:ratings] ||= {}).keys
-    @movies = Movie.find_all_by_rating(@selected_ratings, ordering)
+    @selected_ratings = params[:ratings] || session[:ratings] || {}
+
+    if params[:sort] != session[:sort] or params[:ratings] != session[:ratings]
+      session[:sort] = sort
+      session[:ratings] = @selected_ratings
+      redirect_to :sort => sort, :ratings => @selected_ratings and return
+    end
+    @movies = Movie.find_all_by_rating(@selected_ratings.keys, ordering)
   end
 
   def new

app/views/movies/index.html.haml

@@ -1,19 +1,21 @@
 -#  This file is app/views/movies/index.html.haml
 %h1 All Movies
 
-= form_tag movies_path, :method => :get do
+= form_tag movies_path, :method => :get, :id => 'ratings_form' do
+  = hidden_field_tag "title_sort", true if @title_header
+  = hidden_field_tag ":release_date_sort", true if @date_header
   Include: 
   - @all_ratings.each do |rating|
     = rating
     = check_box_tag "ratings[#{rating}]", 1, @selected_ratings.include?(rating)
-  = submit_tag 'Refresh'
+  = submit_tag 'Refresh', :id => 'ratings_submit'
 
 %table#movies
   %thead
     %tr
-      %th{:class => @title_header}= link_to 'Movie Title', movies_path(:title_sort => true)
+      %th{:class => @title_header}= link_to 'Movie Title', movies_path(:sort => 'title', :ratings => @selected_ratings), :id => 'title_header'
       %th Rating
-      %th{:class=>@date_header}= link_to 'Release Date', movies_path(:release_date_sort => true)
+      %th{:class => @date_header}= link_to 'Release Date', movies_path(:sort => 'release_date', :ratings => @selected_ratings), :id => 'release_date_header'
       %th More Info
   %tbody
     - @movies.each do |movie|