protected function get_model_index( $pk ) {

		if ( isset( $this->pk_map[ $pk ] ) ) {
			return $this->pk_map[ $pk ];
		}

		foreach ( $this->elements as $i => $element ) {

			if ( $this->saver->get_pk( $element ) === $pk ) {

				$this->pk_map[ $pk ] = $i;

				return $i;
			}
		}

		return - 1;
	}