Merge pull request #98 from ThommyH/add_index_submissions

added index to submissions for better performance in recommending exe…

db/migrate/20170323130756_add_index_to_submissions.rb

@@ -0,0 +1,6 @@
+class AddIndexToSubmissions < ActiveRecord::Migration
+  def change
+    add_index :submissions, :exercise_id
+    add_index :submissions, :user_id
+  end
+end