diff --git a/app/models/problem.rb b/app/models/problem.rb
--- a/app/models/problem.rb
+++ b/app/models/problem.rb
@@ -4,10 +4,10 @@
   has_and_belongs_to_many :contests, :uniq => true
 
   #has_and_belongs_to_many :groups
-  has_many :groups_problems, class_name: GroupProblem
+  has_many :groups_problems, class_name: 'GroupProblem'
   has_many :groups, :through => :groups_problems
 
-  has_many :problems_tags, class_name: ProblemTag
+  has_many :problems_tags, class_name: 'ProblemTag'
   has_many :tags, through: :problems_tags
 
   has_many :test_pairs, :dependent => :delete_all