Validate presence of a model, not id

app/models/comment.rb

@@ -1,8 +1,8 @@
 class Comment < ActiveRecord::Base
 
   validates :body, :presence => true
-  validates :asciicast_id, :presence => true
-  validates :user_id, :presence => true
+  validates :asciicast, :presence => true
+  validates :user, :presence => true
 
   belongs_to :user
   belongs_to :asciicast, :counter_cache => true