Clear SG of participant if not attending

app/models/participant.rb

   belongs_to :activity
   belongs_to :subgroup, optional: true
 
+  after_validation :clear_subgroup, if: 'self.attending != true'
+
   validates :person_id,
     uniqueness: {
       scope: :activity_id,
     ParticipantMailer.subgroup_notification(self.person, self.activity, self).deliver_later
   end
 
+  # Clear subgroup if person is set to 'not attending'.
+  def clear_subgroup
+    self.subgroup = nil
+  end
+
 end