code: def validate_fields_only(self, val):
          
          for field_name in self.definition._all_field_names_:
              if not hasattr(val, field_name):
                  raise ValidationError("missing required field '%s'" %
                                        field_name)
docs: To pass field validation, no required field should be missing.
  
          This method assumes that the contents of each field have already been
          validated on assignment, so it's merely a presence check.
  
          FIXME(kelkabany): Since the definition object does not maintain a list
          of which fields are required, all fields are scanned.