protected function addUniqueConstraint(Table $table)
    {
        $unique = new Unique($this->getColumnForParameter('slug_column'));
        $unique->setName($table->getCommonName() . '_slug');
        $unique->addColumn($table->getColumn($this->getParameter('slug_column')));
        if ($this->getParameter('scope_column')) {
            $unique->addColumn($table->getColumn($this->getParameter('scope_column')));
        }
        $table->addUnique($unique);
    }