simplified some Active Record queries

app/controllers/errors_controller.rb

@@ -2,7 +2,7 @@ class ErrorsController < ApplicationController
   before_action :set_execution_environment
 
   def authorize!
-    authorize(@error || Error.where(execution_environment_id: @execution_environment.id))
+    authorize(@error || @execution_environment.errors)
   end
   private :authorize!
 

app/controllers/exercises_controller.rb

@@ -71,7 +71,7 @@ class ExercisesController < ApplicationController
   private :handle_file_uploads
 
   def implement
-    @submission = Submission.where(exercise_id: @exercise.id, user_id: current_user.id).order('created_at DESC').first
+    @submission = current_user.submissions.where(exercise_id: @exercise.id).order('created_at DESC').first
     @files = (@submission ? @submission.collect_files : @exercise.files).select(&:visible).sort_by(&:name_with_extension)
     @paths = collect_paths(@files)
   end

app/controllers/hints_controller.rb

@@ -28,7 +28,7 @@ class HintsController < ApplicationController
   private :hint_params
 
   def index
-    @hints = Hint.where(execution_environment_id: @execution_environment.id).order(:name)
+    @execution_environment.hints.order(:name)
     authorize!
   end