fixes #1546

app/models/forum_post.rb

@@ -7,7 +7,8 @@ class ForumPost < ActiveRecord::Base
   before_validation :initialize_creator, :on => :create
   before_validation :initialize_updater
   before_validation :initialize_is_deleted, :on => :create
-  after_create :update_topic_updated_at
+  after_create :update_topic_updated_at_on_create
+  after_destroy :update_topic_updated_at_on_destroy
   validates_presence_of :body, :creator_id
   validate :validate_topic_is_unlocked
   before_destroy :validate_topic_is_unlocked
@@ -87,7 +88,7 @@ class ForumPost < ActiveRecord::Base
     creator_id == user.id || user.is_janitor?
   end
 
-  def update_topic_updated_at
+  def update_topic_updated_at_on_create
     if topic
       topic.updater_id = CurrentUser.id
       topic.response_count = topic.response_count + 1
@@ -95,6 +96,10 @@ class ForumPost < ActiveRecord::Base
     end
   end
 
+  def update_topic_updated_at_on_destroy
+    topic.update_column(:updated_at, ForumPost.where(:topic_id => topic.id).maximum(:updated_at))
+  end
+
   def initialize_creator
     self.creator_id = CurrentUser.id
   end

test/unit/forum_post_test.rb

@@ -18,9 +18,11 @@ class ForumPostTest < ActiveSupport::TestCase
       setup do
         Danbooru.config.stubs(:posts_per_page).returns(3)
         @posts = []
-        10.times do
+        9.times do
           @posts << FactoryGirl.create(:forum_post, :topic_id => @topic.id, :body => rand(100_000))
         end
+        sleep 2
+        @posts << FactoryGirl.create(:forum_post, :topic_id => @topic.id, :body => rand(100_000))
       end
 
       should "know which page it's on" do
@@ -29,6 +31,12 @@ class ForumPostTest < ActiveSupport::TestCase
         assert_equal(2, @posts[5].forum_topic_page)
         assert_equal(3, @posts[6].forum_topic_page)
       end
+
+      should "update the topic's updated_at when deleted" do
+        @posts.last.destroy
+        @topic.reload
+        assert_equal(@posts[0].updated_at.to_s, @topic.updated_at.to_s)
+      end
     end
 
     context "belonging to a locked topic" do