code: def _key_to_pb(self, pb):
      
      key = self._key
      if key is None:
        pairs = [(self._get_kind(), None)]
        ref = key_module._ReferenceFromPairs(pairs, reference=pb.mutable_key())
      else:
        ref = key.reference()
        pb.mutable_key().CopyFrom(ref)
      group = pb.mutable_entity_group()  # Must initialize this.
      # To work around an SDK issue, only set the entity group if the
      # full key is complete.  TODO: Remove the top test once fixed.
      if key is not None and key.id():
        elem = ref.path().element(0)
        if elem.id() or elem.name():
          group.add_element().CopyFrom(elem)
docs: Internal helper to copy the key into a protobuf.