public function getAddForeignKeyDDL(ForeignKey $fk)
    {
        if ($fk->isSkipSql() || $fk->isPolymorphic()) {
            return;
        }
        $pattern = "
ALTER TABLE %s ADD %s;
";

        return sprintf($pattern,
            $this->quoteIdentifier($fk->getTable()->getName()),
            $this->getForeignKeyDDL($fk)
        );
    }