Check polymorphic type in validations

* Previously, some models had "duplicated" `belongs_to` associations, which were now removed (and replaced by the `include Creation`).

app/models/study_group_membership.rb

@@ -1,13 +1,13 @@
 # frozen_string_literal: true
 
 class StudyGroupMembership < ApplicationRecord
-  ROLES = %w[learner teacher].freeze
-
-  belongs_to :user, polymorphic: true
+  include Creation
   belongs_to :study_group
 
   before_save :destroy_if_empty_study_group_or_user
 
+  ROLES = %w[learner teacher].freeze
+
   def destroy_if_empty_study_group_or_user
     destroy if study_group.blank? || user.blank?
   end