Support for tracking the count of unique participants in a topic

app/models/topic_featured_users.rb

@@ -14,13 +14,14 @@ class TopicFeaturedUsers
     topic.reload unless rails4?
     clear
     update keys(args)
+    update_participant_count
     topic.save
   end
 
   def user_ids
-    [topic.featured_user1_id, 
+    [topic.featured_user1_id,
-     topic.featured_user2_id, 
+     topic.featured_user2_id,
-     topic.featured_user3_id, 
+     topic.featured_user3_id,
      topic.featured_user4_id].uniq.compact
   end
 
@@ -48,4 +49,8 @@ class TopicFeaturedUsers
         topic.send("featured_user#{i+1}_id=", user_id)
       end
     end
+
+    def update_participant_count
+      topic.participant_count = topic.posts.count('distinct user_id')
+    end
 end

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

spec/jobs/feature_topic_users_spec.rb

@@ -42,4 +42,33 @@ describe Jobs::FeatureTopicUsers do
 
   end
 
+  context "participant count" do
+
+    let!(:post) { create_post }
+    let(:topic) { post.topic }
+
+
+    it "it works as expected" do
+
+      # It has 1 participant after creation
+      topic.participant_count.should == 1
+
+      # It still has 1 after featuring
+      Jobs::FeatureTopicUsers.new.execute(topic_id: topic.id)
+      topic.reload.participant_count.should == 1
+
+      # If the OP makes another post, it's still 1.
+      create_post(topic: topic, user: post.user)
+      Jobs::FeatureTopicUsers.new.execute(topic_id: topic.id)
+      topic.reload.participant_count.should == 1
+
+      # If another users posts, it's 2.
+      create_post(topic: topic, user: Fabricate(:evil_trout))
+      Jobs::FeatureTopicUsers.new.execute(topic_id: topic.id)
+      topic.reload.participant_count.should == 2
+
+    end
+
+  end
+
 end