<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _ids(self):
 """The list of primary keys to validate against."""

        for pk in self._pks:
            yield getattr(self, pk)

        for pk in self._pks:

            try:
                yield str(getattr(self, pk))
            except ValueError:
                pass