docs: def check_unique(self):
          """Check the user's email is unique"""
          emails = yield self.view.values(key=self.email)
          user_id = getattr(self, 'id', None)
          users = {x for x in emails if x != user_id and x}
  
          if users:
              raise exceptions.ValidationError(
                  "User with email '{}' already exists".format(self.email))