public function toKeyValue($keyColumn = 'PrimaryKey', $valueColumn = null)
    {
        $ret = [];
        $keyGetterMethod = 'get' . $keyColumn;
        $valueGetterMethod = (null === $valueColumn) ? '__toString' : ('get' . $valueColumn);
        foreach ($this as $obj) {
            $ret[$obj->$keyGetterMethod()] = $obj->$valueGetterMethod();
        }

        return $ret;
    }