public function getUnmappedKeys()
    {
        $u = [];
        $f = $this->unmappedForeignKeys;
        $l = $this->unmappedLocalKeys;
        if (!empty($f)) {
            $u['foreign'] = $f;
        }
        if (!empty($l)) {
            $u['local'] = $l;
        }

        return $u;
    }