caching comment counts

app/models/comment.rb

@@ -8,7 +8,7 @@ class Comment < ActiveRecord::Base
   # want user to vote on the quality of comments.
   #acts_as_votable
 
-  belongs_to :commentable, polymorphic: true
+  belongs_to :commentable, counter_cache: true, polymorphic: true
 
   # NOTE: Comments belong to a user
   belongs_to :user

app/views/admin/events/index.html.haml

@@ -184,4 +184,4 @@
                 %ul.dropdown-menu{ role: 'menu' }
                   = render 'change_state_dropdown', event: event
             %td.text-center
-              = link_to "#{event.comment_threads.count}", admin_conference_program_event_path(@conference.short_title, event), anchor: 'comments-div'
+              = link_to "#{event.comments_count}", admin_conference_program_event_path(@conference.short_title, event), anchor: 'comments-div'

db/migrate/20161229080315_add_comments_count_to_events.rb

@@ -0,0 +1,10 @@
+class AddCommentsCountToEvents < ActiveRecord::Migration
+  def change
+    add_column :events, :comments_count, :integer, default: 0, null: false
+
+    Event.find_each do |event|
+      comments_count = event.comment_threads.count
+      event.update_attribute(:comments_count, comments_count) unless comments_count.zero?
+    end
+  end
+end

db/schema.rb

@@ -242,6 +242,7 @@ ActiveRecord::Schema.define(version: 20170629232817) do
     t.boolean  "is_highlight",                 default: false
     t.integer  "program_id"
     t.integer  "max_attendees"
+    t.integer  "comments_count",               default: 0,     null: false
   end
 
   create_table "events_registrations", force: :cascade do |t|

spec/models/event_spec.rb

@@ -98,6 +98,29 @@ describe Event do
     end
   end
 
+  describe '#comments_count' do
+    context 'has a valid counter cache' do
+      before do
+        create(:comment, commentable: event)
+      end
+
+      it 'successfully increments comments_count' do
+        expected = expect do
+          create(:comment, commentable: event)
+        end
+        expected.to change { event.comments_count }.by(1)
+      end
+
+      it 'successfully decrements comments_count' do
+        expected = expect do
+          event.comment_threads.last.destroy
+          event.reload
+        end
+        expected.to change { event.comments_count }.by(-1)
+      end
+    end
+  end
+
   describe 'scope ' do
     context 'confirmed' do
       it 'returns only confirmed events' do