public function getModifyTableForeignKeysDDL(TableDiff $tableDiff)
    {
        $ret = '';

        foreach ($tableDiff->getRemovedFks() as $fk) {
            $ret .= $this->getDropForeignKeyDDL($fk);
        }

        foreach ($tableDiff->getAddedFks() as $fk) {
            $ret .= $this->getAddForeignKeyDDL($fk);
        }

        foreach ($tableDiff->getModifiedFks() as $fkModification) {
            list($fromFk, $toFk) = $fkModification;
            $ret .= $this->getDropForeignKeyDDL($fromFk);
            $ret .= $this->getAddForeignKeyDDL($toFk);
        }

        return $ret;
    }