Add counter cache for comments on posts

app/models/comment.rb

@@ -39,6 +39,14 @@ class Comment < ActiveRecord::Base
     self.post.touch
   end
 
+  after_create do
+    self.parent.update_comments_counter
+  end
+
+  after_destroy do
+    self.parent.update_comments_counter
+  end
+
   def diaspora_handle
     self.author.diaspora_handle
   end

app/models/like.rb

@@ -24,11 +24,11 @@ class Like < ActiveRecord::Base
   validates :target, :presence => true
 
   after_create do
-    self.target.update_likes_counter
+    self.parent.update_likes_counter
   end
 
   after_destroy do
-    self.target.update_likes_counter
+    self.parent.update_likes_counter
   end
 
   def diaspora_handle

app/models/post.rb

@@ -126,5 +126,11 @@ class Post < ActiveRecord::Base
   def last_three_comments
     self.comments.order('created_at DESC').limit(3).includes(:author => :profile).reverse
   end
+
+  # @return [Integer]
+  def update_comments_counter
+    self.class.where(:id => self.id).
+      update_all(:comments_count => self.comments.count)
+  end
 end
 

db/migrate/20110911213207_counter_cache_on_post_comments.rb.rb

@@ -0,0 +1,14 @@
+class CounterCacheOnPostComments < ActiveRecord::Migration
+  class Post < ActiveRecord::Base; end
+  def self.up
+    add_column :posts, :comments_count, :integer, :default => 0
+    execute <<SQL if Post.count > 0
+      UPDATE posts
+      SET posts.comments_count = (SELECT COUNT(*) FROM comments WHERE comments.post_id = posts.id)
+SQL
+  end
+
+  def self.down
+    remove_column :posts, :comments_count
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20110907205720) do
+ActiveRecord::Schema.define(:version => 20110911213207) do
 
   create_table "aspect_memberships", :force => true do |t|
     t.integer  "aspect_id",  :null => false
@@ -282,6 +282,7 @@ ActiveRecord::Schema.define(:version => 20110907205720) do
     t.string   "root_guid",             :limit => 30
     t.string   "status_message_guid"
     t.integer  "likes_count",                         :default => 0
+    t.integer  "comments_count",                      :default => 0
   end
 
   add_index "posts", ["author_id"], :name => "index_posts_on_person_id"

spec/models/comment_spec.rb

@@ -42,7 +42,6 @@ describe Comment do
     end
   end
 
-
   describe 'User#comment' do
     it "should be able to comment on one's own status" do
       alice.comment("Yeah, it was great", :post => @status)
@@ -53,6 +52,7 @@ describe Comment do
       bob.comment("sup dog", :post => @status)
       @status.reload.comments.first.text.should == "sup dog"
     end
+
     it 'does not multi-post a comment' do
       lambda {
         alice.comment 'hello', :post => @status
@@ -60,6 +60,16 @@ describe Comment do
     end
   end
 
+  describe 'counter cache' do
+    it 'increments the counter cache on its post' do
+      lambda {
+        alice.comment("oh yeah", :post => @status)
+      }.should change{
+        @status.reload.comments_count
+      }.by(1)
+    end
+  end
+
   describe 'xml' do
     before do
       @commenter = Factory.create(:user)
@@ -69,19 +79,24 @@ describe Comment do
       @comment = @commenter.comment "Fool!", :post => @post
       @xml = @comment.to_xml.to_s
     end
+
     it 'serializes the sender handle' do
       @xml.include?(@commenter.diaspora_handle).should be_true
     end
+
     it 'serializes the post_guid' do
       @xml.should include(@post.guid)
     end
+
     describe 'marshalling' do
       before do
         @marshalled_comment = Comment.from_xml(@xml)
       end
+
       it 'marshals the author' do
         @marshalled_comment.author.should == @commenter.person
       end
+
       it 'marshals the post' do
         @marshalled_comment.post.should == @post
       end