counter cache for issue

app/models/issue.rb

@@ -19,7 +19,7 @@ class Issue < ActiveRecord::Base
   include Redmine::SafeAttributes
   include Redmine::Utils::DateCalculation
 
-  belongs_to :project
+  belongs_to :project, :counter_cache => true
   belongs_to :tracker
   belongs_to :status, :class_name => 'IssueStatus', :foreign_key => 'status_id'
   belongs_to :author, :class_name => 'User', :foreign_key => 'author_id'