public function orderBy( $field, $dir = Filter::SORT_ASC ) {
	
		$field = trim($field);

		if( empty($field) )
			throw new \InvalidArgumentException('No field specified');

		$modifier = substr($field, 0, 1);

		if( in_array($modifier, ['+', '-']) ) {
			$dir = ($modifier == '+');
			$field = substr($field, 1);
		}

		$this->orderby[$field] = $dir;

		return $this;

	}