Display hierarchical subterms of custom taxonomy based on depth

Question

There is the following hierarchical taxonomy structure of a specific post:

• Term A
  • SubTerm A.1
  • SubTerm A.2
    • SubTerm A.2.1
• Term B
  • SubTerm B.1
    • SubTerm B.1.1
    • SubTerm B.1.2

How to display

1. all parent terms (result: A, B)
2. all second level terms (result: A.1, A.2, B.1)
3. all third level terms (result: A.2.1, B.1.1, B.1.2)

Answer 1

I love a good challenge!

The function here:

function get_post_categories_sorted ( $post_id ){

    $terms = wp_get_post_terms( $post_id, 'category' );

    $sorted_terms = [];

    foreach( $terms as $term ){

        $depth = count( get_ancestors( $term->term_id, 'category' ) );

        if( ! array_key_exists( $depth, $sorted_terms ) ){
            $sorted_terms[$depth] = [];
        }

        $sorted_terms[$depth][] = $term;

    }

    return $sorted_terms;

}

will give you an array with the structure in the following image. $sorted_terms[0] contains the top level terms, $sorted_terms[1] all the second level term and so on.

It would definitely be worth using a cache or the transients API to store the data as it could get expensive with a lot of terms to sort through as the call to get_ancestors involves a call back to the database for each level it has to traverse back up the chain.

EDIT

You can use this something like the following:

$sorted_terms = get_post_categories_sorted( get_the_ID() );

foreach( $sorted_terms as $key => $level_items ){

    $number = $key + 1;

    echo sprintf( '<p>Level %s Categories</p>', $number );

    echo '<ul>';

        foreach( $level_items as $term ){
            echo sprintf( '<li>%s</li>', $term->name );
        }

    echo '</ul>';

}

EDIT 2

To just use a specific level, you can simplify the code above like this:

$sorted_terms = get_post_categories_sorted( get_the_ID() );

echo '<p>Level 1 Categories</p>';

echo '<ul>';
    //Change the 0 here to whatever level you need, 
    //just remember is is 0 based; IE the first level is 0, 2nd level is 1 etc.
    //If the number doesn't exist, PHP will throw an undefined index error.

    foreach( $sorted_items[0] as $term ){ 
        echo sprintf( '<li>%s</li>', $term->name );
    }

echo '</ul>';

Answer 2

Found the easiest solution with the get_the_terms() | Function. I really do not have a clue why the figures of level 2 ($terms[2]) and 3 ($terms[1]) are not in the correct order, but at least it is realized with a Wordpress function.

Level 1:

<?php
$terms = get_the_terms( $post->ID, 'custom_taxonomy' );
echo $terms[0]->name;
?>  

Level 2:

<?php
$terms = get_the_terms( $post->ID, 'custom_taxonomy' );
echo $terms[2]->name;
?>  

Level3:

<?php
$terms = get_the_terms( $post->ID, 'custom_taxonomy' );
echo $terms[1]->name;
?>  

Answer 3

This code is partially working, in this example displaying the terms of third level with 'depth' => 3. Unfortunately the parent terms are listed too, how is it possible to exclude the parent terms?

<?php function wpse244577_list_terms( $taxonomy, $separator = ', ' ) {        
    // Get the term IDs assigned to post.
    $post_terms = wp_get_object_terms( get_the_ID(), $taxonomy, [ 'fields' => 'ids' ] );
    if ( ! empty( $post_terms ) && ! is_wp_error( $post_terms ) ) {
        $terms = wp_list_categories( [
                'title_li'  => '',
                'style'     => 'none',
                'echo'      => false,
                'taxonomy'  => $taxonomy,
                'include'   => $post_terms,
                'separator' => $separator,
                'depth'  => 3,
        ] );
        $terms = rtrim( trim( str_replace( $separator, $separator, $terms ) ), ', ' );
        $terms = explode( $separator, $terms );
        $terms = array_reverse( $terms );
        $terms = implode( $separator, $terms );
        echo  $terms;
    }
} wpse244577_list_terms( 'custom_taxonomy' ); ?>

Answer 4

Here is an alternative solution. The following function takes an array of arguments, with two possible keys:

• taxonomy: A string. The name of the Taxonomy.
• level: An int. Use 0 for the root elements, 1 for the children of the root elements, and so on.

It returns an array of WP_Term objects.

function wpse_259499_get_subterms( $args = array() ) {
    $args = wp_parse_args( $args, array( 'taxonomy' => 'category', 'level' => 0 ) );

    $subterms = get_transient( 'taxonomy-subterms-' . $args['taxonomy'] . '-' . $args['level'] );
    $terms_ids = array();

    if ( is_array( $subterms ) ) {
        return $subterms;
    }

    if ( 0 == $args['level'] ) {
        $terms = get_terms( array(
            'taxonomy' => $args['taxonomy'],
            'get' => 'all',
            'orderby' => 'id',
            'fields' => 'id=>parent',
        ) );

        foreach ( $terms as $term_id => $parent ) {
            if ( ! $parent ) {
                $terms_ids[] = $term_id;
            }
        }
    } else if ( $hierarchy = _get_term_hierarchy( $args['taxonomy'] ) ) {
        $subterms_args = array( 'taxonomy' => $args['taxonomy'], 'level' => $args['level'] - 1 );

        foreach ( wpse_259499_get_subterms( $subterms_args ) as $term ) {
            if ( isset( $hierarchy[ $term->term_id ] ) ) {
                $terms_ids = array_merge( $terms_ids, $hierarchy[ $term->term_id ] );
            }
        }
    }

    if ( $terms_ids ) {
        $subterms = get_terms( array(
            'taxonomy' => $args['taxonomy'],
            'get' => 'all',
            'include' => $terms_ids,
        ) );
    } else {
        $subterms = array();
    }

    set_transient( 'taxonomy-subterms-' . $args['taxonomy'] . '-' . $args['level'], $subterms, HOUR_IN_SECONDS );

    return $subterms;
}

Note 1: If you decide to rename the function, please note that the function is calling itself, so you need to change the name in the recursive call as well.

Note 2: The results are being stored in a transient for an hour. In order to get fresh results during development, you can add the following line at the beginning of the function:

delete_transient( 'taxonomy-subterms-' . $args['taxonomy'] . '-' . $args['level'] );

You may also need to delete those transients every time a category in the taxonomy of interest is added, deleted or modified, if you want to have accurate results all the time.

Example

The following code can be used in a template, to generate an unordered list of links to all terms in the selected level:

<ul>
<?php foreach ( wpse_259499_get_subterms( array( 'taxonomy' => 'category', 'level' => 0 ) ) as $term ): ?>
    <li><a href="<?php echo esc_attr( get_term_link( $term ) ); ?>"><?php echo esc_html( $term->name ); ?></a></li>
<?php endforeach; ?>
</ul>