diff --git a/app/models/request_for_comment.rb b/app/models/request_for_comment.rb
index f0f2953f..fc77352c 100644
--- a/app/models/request_for_comment.rb
+++ b/app/models/request_for_comment.rb
@@ -8,7 +8,8 @@ class RequestForComment < ActiveRecord::Base
   has_many :subscriptions
 
   scope :unsolved, -> { where(solved: [false, nil]) }
-  scope :not_stale, -> { where("user_id%10 < 3").where(exercise.exercise_collections.none{|ec| ec.id = 3}) }
+  scope :stale, -> { where(exercise.exercise_collections.any{|ec| ec.id = 3}).where("user_id%10 < 3")}
+  scope :not, ->(scope) { where(scope.where_values.reduce(:and).not) }
 
     def self.last_per_user(n = 5)
       from("(#{row_number_user_sql}) as request_for_comments")
diff --git a/app/models/submission.rb b/app/models/submission.rb
index 496ae056..74ffcd24 100644
--- a/app/models/submission.rb
+++ b/app/models/submission.rb
@@ -66,6 +66,6 @@ class Submission < ActiveRecord::Base
   end
 
   def unsolved_rfc
-    RequestForComment.unsolved.not_stale.where(exercise_id: exercise).where.not(question: nil).order("RANDOM()").find { | rfc_element |(rfc_element.comments_count < MAX_COMMENTS_ON_RECOMMENDED_RFC) }
+    RequestForComment.unsolved.not(:stale).where(exercise_id: exercise).where.not(question: nil).order("RANDOM()").find { | rfc_element |(rfc_element.comments_count < MAX_COMMENTS_ON_RECOMMENDED_RFC) }
   end
 end