rewrote algorithm in SQL for better performance

app/controllers/recommendations_controller.rb

@@ -16,7 +16,7 @@ def index
       end
 
       format.json do
-        subtopic_ids = @recommendations.map &:first
+        subtopic_ids = @recommendations.map { |r| r["subtopic_id"] }
         render json: subtopic_ids
       end
     end

app/models/user.rb

@@ -17,3 +17,14 @@ class User < ActiveRecord::Base
     where listens: { subtopic_id: subtopic_id }
   }
 end
+
+<<-SQL
+SELECT subtopic_id, COUNT(id) AS listens_count FROM listens
+WHERE listens.user_id IN (
+  SELECT users.user_id FROM users
+  RIGHT JOIN listens ON listens.user_id = users.user_id
+  WHERE listens.subtopic_id = '575f6209a06acf0300140150'
+)
+GROUP BY subtopic_id
+ORDER BY listens_count DESC
+SQL

app/services/recommender.rb

@@ -4,32 +4,17 @@ def initialize(subtopic_id)
   end
 
   def solve
-    if (r = Recommendation.find_by_subtopic_id @subtopic_id)
-      r.recommended_subtopic_ids
-    else
-      sub_topics = get_sub_topics
-      recommendation = create_recommendation_for sub_topics
-      recommendation.recommended_subtopic_ids
-    end
-  end
-
-  private
-
-  def get_sub_topics
-    User.who_listened_to(@subtopic_id).inject({}) do |sub_topics, user|
-      user.listens.pluck(:subtopic_id).each do |subtopic_id|
-        next if subtopic_id == @subtopic_id
-        sub_topics[subtopic_id] ||= 0
-        sub_topics[subtopic_id] += 1
-      end
-      sub_topics
-    end.sort_by { |subtopic_id, listens_count| listens_count }.reverse
-  end
-
-  def create_recommendation_for(sub_topics)
-    Recommendation.find_or_create_by(subtopic_id: @subtopic_id).tap do |r|
-      r.recommended_subtopic_ids = sub_topics.map { |sid, listens_count| [sid, listens_count] }
-      r.save
-    end
+    results = ActiveRecord::Base.connection.execute <<-SQL.strip_heredoc
+      SELECT subtopic_id, COUNT(id) AS listens_count FROM listens
+      WHERE listens.user_id IN (
+        SELECT users.user_id FROM users
+        RIGHT JOIN listens ON listens.user_id = users.user_id
+        WHERE listens.subtopic_id = '#{@subtopic_id}'
+      ) 
+      AND listens.subtopic_id != '#{@subtopic_id}'
+      GROUP BY subtopic_id
+      ORDER BY listens_count DESC
+    SQL
+    results.to_a
   end
 end

app/views/recommendations/index.html.haml

@@ -10,10 +10,10 @@
         %th SubTopic
         %th Listens
     %tbody
-      - @recommendations.each do |subtopic_id, listens|
-        - sub_topic = SubTopic.find_by_subtopic_id subtopic_id
+      - @recommendations.each do |recommendation|
+        - sub_topic = SubTopic.find_by_subtopic_id recommendation["subtopic_id"]
         %tr
           %td= link_to sub_topic.name, sub_topic
-          %td= listens
+          %td= recommendation["listens_count"]
 
 = paginate @recommendations

lib/tasks/load.rake

@@ -48,42 +48,12 @@ namespace :load do
     puts "\aDone."
   end
 
-  desc "Generate recommendations from sub_topics"
-  task recommendations: :environment do
-    subtopic_ids = SubTopic.pluck :subtopic_id
-    total = subtopic_ids.size
-    puts "Creating #{total} recommendations..."
-
-    subtopic_ids.each_with_index do |subtopic_id, i|
-      subtopic = SubTopic.find_by_subtopic_id subtopic_id
-      puts "\tGenerating recommendations for \"#{subtopic.name}\". #{i+1} of #{total} (#{percent_of i, total}%) done."
-
-      Recommender.new(subtopic_id).solve
-    end
-    puts "\aDone."
-  end
-
   desc "Delete and reload SubTopics, Listens, and Users"
   task reload: :environment do
     puts "Deleting all data..."
-    [Recommendation, Listen, SubTopic, User].map &:delete_all
+    [Listen, SubTopic, User].map &:delete_all
 
     puts "Loading all data..."
     %w[sub_topics listens users].each { |task| Rake::Task["load:#{task}"].invoke }
   end
-
-  desc "Clear listens, sub_topics, and users tables"
-  task reset: :environment do
-    Rake::Task["load:reload"].invoke
-    Rake::Task["load:recommendations"].invoke
-  end
-
-  desc "Clear all recommendations"
-  task clear_recommendations: :environment do
-    Recommendation.delete_all
-  end
-
-  def percent_of(is, of)
-    sprintf "%.2f", (is + 1).to_f / of.to_f * 100.0
-  end
 end