Support for tracking the count of unique participants in a topic

db/migrate/20131114185225_add_participant_count_to_topics.rb

@@ -0,0 +1,13 @@
+class AddParticipantCountToTopics < ActiveRecord::Migration
+  def up
+    add_column :topics, :participant_count, :integer, default: 1
+
+    execute "UPDATE topics SET participant_count =
+              (SELECT COUNT(DISTINCT p.user_id) FROM posts AS p WHERE p.topic_id = topics.id)"
+  end
+
+  def down
+    remove_column :topics, :participant_count
+  end
+
+end