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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validate_fields_only(self, val):
 """ 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. """

        for field_name in self.definition._all_field_names_:
            if not hasattr(val, field_name):
                raise ValidationError("missing required field '%s'" %
                                      field_name)