public function orderBy(array $order)
    {
        $orderBy = ['keys' => [], 'directions' => []];
        
        foreach ($order as $key => $direction) {
            $orderBy['keys'][] = $key;
            $orderBy['directions'][] = $direction;
        }
        
        $this->_order = $orderBy;
        
        return $this;
    }