Rename PostTimestampChanger -> TopicTimestampChanger.

app/services/topic_timestamp_changer.rb

@@ -0,0 +1,52 @@
+class TopicTimestampChanger
+  def initialize(params)
+    @topic = params[:topic] || Topic.with_deleted.find(params[:topic_id])
+    @posts = @topic.posts
+    @timestamp = Time.at(params[:timestamp])
+    @time_difference = calculate_time_difference
+  end
+
+  def change!
+    ActiveRecord::Base.transaction do
+      last_posted_at = @timestamp
+      now = Time.zone.now
+
+      @posts.each do |post|
+        if post.is_first_post?
+          update_post(post, @timestamp)
+        else
+          new_created_at = Time.at(post.created_at.to_f + @time_difference)
+          new_created_at = now if new_created_at > now
+          last_posted_at = new_created_at if new_created_at > last_posted_at
+          update_post(post, new_created_at)
+        end
+      end
+
+      update_topic(last_posted_at)
+
+      yield(@topic) if block_given?
+    end
+
+    # Burst the cache for stats
+    [AdminDashboardData, About].each { |klass| $redis.del klass.stats_cache_key }
+  end
+
+  private
+
+  def calculate_time_difference
+    @timestamp - @topic.created_at
+  end
+
+  def update_topic(last_posted_at)
+    @topic.update_attributes(
+      created_at: @timestamp,
+      updated_at: @timestamp,
+      bumped_at: @timestamp,
+      last_posted_at: last_posted_at
+    )
+  end
+
+  def update_post(post, timestamp)
+    post.update_attributes(created_at: timestamp, updated_at: timestamp)
+  end
+end