public function modifyTable()
    {
        $table = $this->getTable();

        if (!$table->hasColumn($this->getParameter('slug_column'))) {
            $table->addColumn([
                'name'     => $this->getParameter('slug_column'),
                'type'     => 'VARCHAR',
                'size'     => 255,
                'required' => false,
            ]);
            // add a unique to column
            if ('true' === $this->getParameter('unique_constraint')) {
                $this->addUniqueConstraint($table);
            }
        }
    }