Updates the search index for this instance.

        This happens automatically on put.

def search_update_index(self):
        """
        Updates the search index for this instance.

        This happens automatically on put.
        """
        doc_id = self.search_get_document_id(self.key)

        fields = [search.AtomField('class_name', name) for name in self.search_get_class_names()]

        index = self.search_get_index()

        if self.searchable_fields is None:
            searchable_fields = []

            for field, prop in self._properties.items():
                if field == 'class':
                    continue
                for class_, field_type in SEARCHABLE_PROPERTY_TYPES.items():
                    if isinstance(prop, class_):
                        searchable_fields.append(field)
        else:
            searchable_fields = self.searchable_fields

        for f in set(searchable_fields):
            prop = self._properties[f]
            value = getattr(self, f)
            field = None
            field_found = False
            for class_, field_type in SEARCHABLE_PROPERTY_TYPES.items():
                if isinstance(prop, class_):
                    field_found = True
                    if value is not None:
                        if isinstance(value, list) or isinstance(value, tuple) or isinstance(value, set):
                            for v in value:
                                field = field_type(name=f, value=v)
                        elif isinstance(value, ndb.Key):
                            field = field_type(name=f, value=value.urlsafe())
                        else:
                            field = field_type(name=f, value=value)
            if not field_found:
                raise ValueError('Cannot find field type for %r on %r' % (prop, self.__class__))

            if field is not None:
                fields.append(field)

        document = search.Document(doc_id, fields=fields)
        index.put(document)