Resolve conflict

app/controllers/topics_controller.rb

@@ -6,7 +6,7 @@ class TopicsController < ApplicationController
   # GET /topics
   # GET /topics.xml
   def index
-    @topics = Topic.paginate :per_page => 10, :page => params[:page], :order => "created_at DESC"
+    @topics = Topic.paginate :per_page => 10, :page => params[:page], :order => "updated_at DESC"
 
     respond_to do |format|
       format.html # index.html.erb

app/models/reply.rb

@@ -1,9 +1,19 @@
 class Reply < ActiveRecord::Base
   belongs_to :author, :class_name => "User"
-  belongs_to :topic
+  belongs_to :topic, :counter_cache => true
+
+  after_create :set_topics_last_reply
 
   def mail_subject
     "Re: [Cuniculi Cavum] #{topic.subject}"
   end
 
+  private
+
+    # This has the convenient side-effect of refreshing Topic#updated_at for
+    # the topic index's sorting.
+    def set_topics_last_reply
+      topic.update_attribute :last_reply, self
+    end
+
 end

app/models/topic.rb

@@ -1,5 +1,6 @@
 class Topic < ActiveRecord::Base
   belongs_to :author, :class_name => "User"
+  belongs_to :last_reply, :class_name => "Reply"
   has_many :replies
 
   def mail_subject

app/views/topics/index.html.erb

@@ -3,7 +3,12 @@
   <div class="authorship">
     posted by <%= link_to h(topic.author.name), topic.author %></a>
     <%= time_ago_in_words(topic.created_at) %> ago
-    (<%= topic.replies.count %> <%= topic.replies.count == 1 ? "reply" : "replies" %>)
+    (<%= topic.replies.count -%>
+      <%= topic.replies.count == 1 ? "reply" : "replies" -%>
+      <%- if topic.last_reply -%>, last one by
+        <%= link_to h(topic.last_reply.author.name), topic.last_reply.author %>
+        <%= time_ago_in_words(topic.last_reply.created_at) %> ago
+      <%- end -%>)
   </div>
 
   <h2><%= link_to h(topic.subject), topic %></h2>

db/migrate/20090316152049_add_replies_count_to_topics.rb

@@ -0,0 +1,9 @@
+class AddRepliesCountToTopics < ActiveRecord::Migration
+  def self.up
+    add_column :topics, :replies_count, :integer, :default => 0
+  end
+
+  def self.down
+    remove_column :topics, :replies_count
+  end
+end

db/migrate/20090330040325_add_last_reply_to_topics.rb

@@ -0,0 +1,9 @@
+class AddLastReplyToTopics < ActiveRecord::Migration
+  def self.up
+    add_column :topics, :last_reply_id, :integer
+  end
+
+  def self.down
+    remove_column :topics, :last_reply_id
+  end
+end

db/schema.rb

@@ -75,6 +75,8 @@
     t.text     "body"
     t.datetime "created_at"
     t.datetime "updated_at"
+    t.integer  "replies_count", :default => 0
+    t.integer  "last_reply_id"
   end
 
   create_table "users", :force => true do |t|