def pre_save(self, model_instance, add):
value = super(UserField, self).pre_save(model_instance, add)<EOL>if not value and not add:<EOL><INDENT>value = self.get_os_username()<EOL>setattr(model_instance, self.attname, value)<EOL>return value<EOL><DEDENT>return value<EOL>
Updates username created on ADD only.