Add excerpt column to topics table to remove N+1 query in ListableTopicSerializer

app/serializers/listable_topic_serializer.rb

@@ -64,11 +64,6 @@ class ListableTopicSerializer < BasicTopicSerializer
     pinned
   end
 
-  def excerpt
-    # excerpt should be hoisted into topic, this is an N+1 query ... yuck
-    object.posts.by_post_number.first.try(:excerpt, 220, strip_links: true) || nil
-  end
-
   def pinned
     PinnedCheck.new(object, object.user_data).pinned?
   end

db/migrate/20140318150412_add_excerpt_to_topics.rb

@@ -0,0 +1,18 @@
+class AddExcerptToTopics < ActiveRecord::Migration
+  def up
+    add_column :topics, :excerpt, :string, limit: 1000
+
+    topic_ids = execute("SELECT id FROM topics WHERE pinned_at IS NOT NULL").map {|r| r['id'].to_i }
+    topic_ids.each do |topic_id|
+      cooked = execute("SELECT cooked FROM posts WHERE topic_id = #{topic_id} ORDER BY post_number ASC LIMIT 1")[0]['cooked']
+      if cooked
+        excerpt = ExcerptParser.get_excerpt(cooked, 220, strip_links: true)
+        execute "UPDATE topics SET excerpt = #{ActiveRecord::Base.sanitize(excerpt)} WHERE id = #{topic_id}"
+      end
+    end
+  end
+
+  def down
+    remove_column :topics, :excerpt
+  end
+end

lib/post_creator.rb

@@ -188,6 +188,7 @@ class PostCreator
     attrs = {last_posted_at: @post.created_at, last_post_user_id: @post.user_id}
     attrs[:bumped_at] = @post.created_at unless @post.no_bump
     attrs[:word_count] = (@topic.word_count || 0) + @post.word_count
+    attrs[:excerpt] = @post.excerpt(220, strip_links: true) if new_topic?
     @topic.update_attributes(attrs)
   end
 

lib/post_revisor.rb

@@ -14,6 +14,7 @@ class PostRevisor
     @post.acting_user = @user
     revise_post
     update_category_description
+    update_topic_excerpt
     post_process_post
     update_topic_word_counts
     @post.advance_draft_sequence
@@ -107,6 +108,10 @@ class PostRevisor
     end
   end
 
+  def update_topic_excerpt
+    @post.topic.update_column(:excerpt, @post.excerpt(220, strip_links: true)) if @post.post_number == 1
+  end
+
   def post_process_post
     @post.invalidate_oneboxes = true
     @post.trigger_post_process

spec/components/post_creator_spec.rb

@@ -184,6 +184,16 @@ describe PostCreator do
 
         user2.user_stat.reload.topic_reply_count.should == 1
       end
+
+      it 'sets topic excerpt if first post, but not second post' do
+        first_post = creator.create
+        topic = first_post.topic.reload
+        topic.excerpt.should be_present
+        expect {
+          PostCreator.new(first_post.user, topic_id: first_post.topic_id, raw: "this is the second post").create
+          topic.reload
+        }.to_not change { topic.excerpt }
+      end
     end
 
     context 'when auto-close param is given' do

spec/components/post_revisor_spec.rb

@@ -271,5 +271,21 @@ describe PostRevisor do
         end
       end
     end
+
+    describe "topic excerpt" do
+      it "topic excerpt is updated only if first post is revised" do
+        revisor = described_class.new(post)
+        first_post = topic.posts.by_post_number.first
+        expect {
+          revisor.revise!(first_post.user, 'Edit the first post', revised_at: first_post.updated_at + 10.seconds)
+          topic.reload
+        }.to change { topic.excerpt }
+        second_post = Fabricate(:post, post_args.merge(post_number: 2, topic_id: topic.id))
+        expect {
+          described_class.new(second_post).revise!(second_post.user, 'Edit the 2nd post')
+          topic.reload
+        }.to_not change { topic.excerpt }
+      end
+    end
   end
 end