public function group_by( $attribute )
	{
		$new = new self();

		for ( $i = 0; $i < count( $this ); ++$i ) {

			$key = (string)$this[$i]->$attribute; 

			if ( ! isset( $new[$key] ) )
				$new[$key] = new self();

			$new[$key][] = $this[$i];

		}

		return $new;
	}