How can I remove a unique constraint on a field when performing a down migration?

Copy-pasted from https://stackoverflow.com/questions/6503...-migration, feel free to answer the question there too to win a few points.

I have to add a unique constraint to a field after the table was created. I therefore set up a migration file with this in the up() method:

Code:

$fields = [
    'slug' => [
        'type' => 'varchar',
        'constraint' => 255,
        'unique' => true
    ]
];
$this->forge->modifyColumn('pages', $fields);

And in down():

Code:

$fields = [
    'slug' => [
        'type' => 'varchar',
        'constraint' => 255,
        'unique' => false
    ]
];
$this->forge->modifyColumn('pages', $fields);

This doesn't work.

When calling

Code:

php spark migrate:rollback

, the migration is correctly un-applied ("migrations" table is modified) but the fields stay unique.
What should I put in down() instead?