code: def _get_pk(self):
          
          pk = None
          if self._lazy_collection['pks']:
              if len(self._lazy_collection['pks']) > 1:
                  raise ValueError('Too much pks !')
              pk = list(self._lazy_collection['pks'])[0]
          return pk
docs: Return None if we don't have any filter on a pk, the pk if we have one,
          or raise a ValueError if we have more than one.
          For internal use only.