diff --git a/app/models/code_ocean/file.rb b/app/models/code_ocean/file.rb
index 9cc85832..ccfa7f46 100644
--- a/app/models/code_ocean/file.rb
+++ b/app/models/code_ocean/file.rb
@@ -24,6 +24,10 @@ module CodeOcean
     scope :editable, -> { where(read_only: false) }
     scope :visible, -> { where(hidden: false) }
 
+    ROLES.each do |role|
+      scope :"#{role}s", -> { where(role: role) }
+    end
+
     validates :feedback_message, if: :teacher_defined_test?, presence: true
     validates :feedback_message, absence: true, unless: :teacher_defined_test?
     validates :file_type_id, presence: true
diff --git a/app/models/exercise.rb b/app/models/exercise.rb
index 5fb887e0..28f9781f 100644
--- a/app/models/exercise.rb
+++ b/app/models/exercise.rb
@@ -43,7 +43,7 @@ class Exercise < ActiveRecord::Base
   private :generate_token
 
   def maximum_score
-    files.where(role: 'teacher_defined_test').sum(:weight)
+    files.teacher_defined_tests.sum(:weight)
   end
 
   def set_default_values
@@ -56,7 +56,7 @@ class Exercise < ActiveRecord::Base
   end
 
   def valid_main_file?
-    if files.where(role: 'main_file').count > 1
+    if files.main_files.count > 1
       errors.add(:files, I18n.t('activerecord.errors.models.exercise.at_most_one_main_file'))
     end
   end