diff --git a/app/models/problem.rb b/app/models/problem.rb
--- a/app/models/problem.rb
+++ b/app/models/problem.rb
@@ -1,4 +1,4 @@
-class Problem < ActiveRecord::Base
+class Problem < ApplicationRecord
 
   belongs_to :description
   has_and_belongs_to_many :contests, :uniq => true