public function find_or_fail( $primary_key ) {

		$result = $this->find( $primary_key );

		if ( is_array( $primary_key ) ) {
			if ( count( $result ) == count( array_unique( $primary_key ) ) ) {
				return $result;
			}
		} elseif ( $result ) {
			return $result;
		}

		throw new ModelNotFoundException( "No model found for '$primary_key'." );
	}