protected function whereCondition(array $conditions, $preserveKeys)
    {
        $key = first_key($conditions);  $value = array_shift($conditions);

        $where = $this->whereRecursive($this->items, $key, $value, $preserveKeys);

        foreach ($conditions as $key => $value)
        {
            $where = $this->whereRecursive($where, $key, $value, $preserveKeys);
        }

        return $where;
    }