diff --git a/app/models/problem.rb b/app/models/problem.rb
--- a/app/models/problem.rb
+++ b/app/models/problem.rb
@@ -2,6 +2,7 @@
 
   belongs_to :description
   has_and_belongs_to_many :contests, :uniq => true
+  has_and_belongs_to_many :groups
   has_many :test_pairs, :dependent => :delete_all
   has_many :testcases, :dependent => :destroy