Disable Attachment Page Except for Category

Question

I'm looking for a solution similar to here, except I want to keep attachment pages for attachments in a specific category.

I tried adding the below function, but it's still redirecting all of them:

add_action( 'template_redirect', 'test_attachment_redirect', 10 );
function test_attachment_redirect() {
    if( is_attachment() && !is_category('keep')) {
        $url = wp_get_attachment_url( get_queried_object_id() );
        wp_redirect( $url, 301 );
    }
    return;
}

For context, my attachments do have categories with this function:

function add_categories_to_attachments() {
    register_taxonomy_for_object_type( 'category', 'attachment' );
}
add_action( 'init' , 'add_categories_to_attachments' );

Answer 1

The problem is that is_category does not do what you thought it does. is_category( 'keep' ) tests that the current page is a category archive for the category keep, it does not check if the current attachment/post is in the keep category.

The official WordPress developers docs for is_category start with:

Determines whether the query is for an existing category archive page.

https://developer.wordpress.org/reference/functions/is_category/

Instead, what you want is to check if the current post has a category assigned, aka has_category:

has_category( string|int|array $category = '', int|WP_Post $post = null ): bool

Checks if the current post has any of given category.

https://developer.wordpress.org/reference/functions/has_category/