Categories Hierarchy

Hi John,

This is possible if the categories have an actual parent/child hierarchy:

Germany – no parent
Communication – parent: Germany
Small Talk – parent: Communication

Then add this PHP snippet:

add_filter( 'get_the_terms', function( $terms, $post_id, $taxonomy ) {
    if (
        is_admin() ||
        'category' !== $taxonomy ||
        ! is_singular( 'post' ) ||
        ! is_array( $terms )
    ) {
        return $terms;
    }

    usort( $terms, function( $a, $b ) {
        $a_depth = count(
            get_ancestors( $a->term_id, 'category', 'taxonomy' )
        );

        $b_depth = count(
            get_ancestors( $b->term_id, 'category', 'taxonomy' )
        );

        // Alphabetical order when terms have the same depth.
        if ( $a_depth === $b_depth ) {
            return strcasecmp( $a->name, $b->name );
        }

        return $a_depth <=> $b_depth;
    } );

    return $terms;
}, 10, 3 );

Adding PHP: https://docs.generatepress.com/article/adding-php/

This changes the output to:
Germany, Communication, Small Talk

All three categories must remain assigned to the post if you want all three displayed. If they are three unrelated categories rather than a real parent/child chain, WordPress has no hierarchy it can use for sorting. In that case, separate taxonomies for Country, Topic, and Subtopic would be the more reliable structure.

Hi John,

I can not see the link without logging into your site.
Can you provide an admin login?

Haha I’m great, thanks for asking 🙂

The code is working, but it is intended to only work on a single post, not the archives.

Do you want to change the order for archives and blog as well?

If so, the code can be updated to this:

add_filter( 'get_the_terms', function( $terms, $post_id, $taxonomy ) {
    if (
        is_admin() ||
        'category' !== $taxonomy ||
        ! is_array( $terms )
    ) {
        return $terms;
    }

    usort( $terms, function( $a, $b ) {
        $a_depth = count( get_ancestors( $a->term_id, 'category', 'taxonomy' ) );
        $b_depth = count( get_ancestors( $b->term_id, 'category', 'taxonomy' ) );

        if ( $a_depth === $b_depth ) {
            return strcasecmp( $a->name, $b->name );
        }

        return $a_depth <=> $b_depth;
    } );

    return $terms;
}, 10, 3 );

You are welcome   🙂