caching comment counts

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