Require pair programming for a given set of exercises

app/controllers/exercises_controller.rb

@@ -315,6 +315,8 @@ class ExercisesController < ApplicationController
       # we are just acting on behalf of a single user who has already worked on this exercise as part of a programming group **in the context of the current study group**
       session[:pg_id] = pg.id
       @current_contributor = pg
+    elsif session[:pg_id].blank? && PairProgramming23Study.participate_in_pp?(current_user, @exercise) && PairProgramming23Study::FORCED_EXERCISE_IDS.include?(@exercise.id)
+      return redirect_back(fallback_location: new_exercise_programming_group_path(@exercise))
     elsif session[:pg_id].blank? && PairProgramming23Study.participate?(current_user, @exercise) && current_user.submissions.where(study_group_id: current_user.current_study_group_id, exercise: @exercise).none?
       Event.find_or_create_by(category: 'pp_work_alone', user: current_user, exercise: @exercise, data: nil, file_id: nil)
       current_user.pair_programming_waiting_users&.find_by(exercise: @exercise)&.update(status: :worked_alone)

app/views/programming_groups/new.html.slim

@@ -30,6 +30,7 @@ h1.d-inline-block = t('programming_groups.new.create_programming_pair')
     .col-12.d-none.d-md-block
       = render('form')
 
+  - unless PairProgramming23Study.participate_in_pp?(current_user, @exercise) && PairProgramming23Study::FORCED_EXERCISE_IDS.include?(@exercise.id)
     .row
       .col-12
         h5 = t('programming_groups.new.work_alone')

lib/pair_programming23_study.rb

@@ -3,26 +3,28 @@
 class PairProgramming23Study
   ENABLE = ENV.fetch('PAIR_PROGRAMMING_23_STUDY', nil) == 'true'
   STUDY_GROUP_IDS = [368, 451].freeze
+  # All easy tasks of the first week to be solved by the participants on their own
+  EXCLUDED_EXERCISE_IDS = [636, 647, 648, 649, 637, 638, 623, 639, 650, 625, 624, 651, 653, 654, 655, 664, 656].freeze
+  # The participants are forced to work in pairs on these tasks
+  FORCED_EXERCISE_IDS = [723].freeze
 
   def self.participate?(user, exercise)
     ENABLE || participate_in_pp?(user, exercise)
   end
 
   def self.participate_in_pp?(user, exercise)
-    # All easy tasks of the first week to be solved by the participants on their own
-    if experiment_course?(user.current_study_group_id) && [636, 647, 648, 649, 637, 638, 623, 639, 650, 625, 624, 651, 653, 654, 655, 664, 656].exclude?(exercise.id)
+    return false unless experiment_course?(user.current_study_group_id)
+    return false if EXCLUDED_EXERCISE_IDS.include?(exercise.id)
     return true if user.external_user? && fixed_enrolled_users.include?([user.consumer_id.to_s, user.external_id])
 
     user_group = user.id % 3 # => 0, 1, 2
     case user_group
       when 0, 1
-          return true
+        true
       else # 2
-          return false
-      end
-    end
         false
     end
+  end
 
   def self.experiment_course?(study_group_id)
     STUDY_GROUP_IDS.include? study_group_id