fixed the query, removed scopes, added paranthesis for clarity.

app/controllers/exercises_controller.rb

@@ -416,7 +416,7 @@ class ExercisesController < ApplicationController
           return
         end
 
-        if @is_experimental_course and @rfc_group == :hide_rfc
+        if @is_experimental_course and (@rfc_group == :hide_rfc)
           redirect_to_lti_return_path
           return
         end

app/models/request_for_comment.rb

@@ -8,8 +8,6 @@ class RequestForComment < ActiveRecord::Base
   has_many :subscriptions
 
   scope :unsolved, -> { where(solved: [false, nil]) }
-  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")

app/models/submission.rb

@@ -66,6 +66,10 @@ 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) }
+    # old query
+    # 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) }
+
+    # experimental query:
+    RequestForComment.unsolved.joins('JOIN exercise_collections_exercises ece ON ece.exercise_id = request_for_comments.exercise_id').where('ece.exercise_collection_id != 3 OR user_id%10 > 3').where.not(question: nil).order("RANDOM()").to_a
   end
 end