public function pluckCallback(Collection $collection, $key, array $matches)
    {
        $indexes = explode(',', $key);
        $result = array();
        foreach ($indexes as $idx) {
            $idx = trim($idx);
            if ($idx != '') {
                if (array_key_exists($idx, $this->contents)) {
                    $result[$idx] = $this->contents[$idx];
                }
            }
        }
        return $result;
    }