Prevent publishing of uncategorized posts

Question

I'd like to prevent posts under the default category named "affitti" to be published. I tried this but with no effect

function prevent_default_category_publish($data, $postarr) {
if (in_category('affitti', $postarr['ID'])) {
    wp_die('Error: Posts cannot be published with the default category.');
}
return $data;
}
add_filter('wp_insert_post_data', 'prevent_default_category_publish', 10, 2);

Answer 1

The in_category function works only on existing (saved) posts. Here you can directly access the category in $postarr like this:

if ('affitti' == $postarr['post_category'])) {
  wp_die('Error: Posts cannot be published with the default category.');
  }

By the way, are you sure you want to use wp_die for this and not some other error message? People may loose their work if WP stops completely when they try to publish.