Reduce SQL queries for exercise batch update

Fixes CODEOCEAN-10D

app/controllers/exercises_controller.rb

@@ -39,12 +39,17 @@ class ExercisesController < ApplicationController
   end
 
   def batch_update
-    @exercises = Exercise.all
+    update_map = {}
+    update_params = params.permit(exercises: %i[id public])
+    update_params[:exercises].each_value do |param|
+      update_map[param[:id]] = param[:public]
+    end
+
+    @exercises = Exercise.where(id: update_map.keys).includes(:execution_environment, :files)
     authorize!
-    @exercises = params[:exercises].values.map do |exercise_params|
-      exercise = Exercise.find(exercise_params.delete(:id))
-      exercise.update(exercise_params.permit(:public))
-      exercise
+
+    @exercises.each do |exercise|
+      exercise.update(public: update_map[exercise.id.to_s])
     end
     render(json: {exercises: @exercises})
   end

spec/controllers/exercises_controller_spec.rb

@@ -14,8 +14,9 @@ RSpec.describe ExercisesController do
   end
 
   describe 'PUT #batch_update' do
-    let(:attributes) { ActionController::Parameters.new(public: 'true').permit! }
-    let(:perform_request) { proc { put :batch_update, params: {exercises: {0 => attributes.merge(id: exercise.id)}} } }
+    let(:attributes) { {public: 'true'} }
+    let(:params) { ActionController::Parameters.new(public: 'true').permit! }
+    let(:perform_request) { proc { put :batch_update, params: {exercises: {0 => params.merge(id: exercise.id)}} } }
 
     before { perform_request.call }