Exclude Custom Post Type from shared Custom Taxonomy

Question

I have a few custom post types and two of them are sharing a custom taxonomy.

Is there a way to list the terms of that taxonomy for only one of the custom post types?

At the moment I can get a list of the terms and display them how I want, but its a mix of both custom post types. I need a way to somehow filter out the other post type.

Heres what I've got in my sidebar.php:

<?php
    $post_type = get_post_type();

    if($post_type == 'artists' || $post_type == 'educations') {
        $taxonomy = 'art_categories'; // Sharing same taxonomy
    } else {
        $taxonomy = $post_type.'_categories'; // Other post type taxonomy
    }

    $term = get_term_by("slug", get_query_var($taxonomy), $taxonomy);

    $children = get_term_children($term->term_id, $taxonomy);


    if(empty($children)) { // Cats with NO kids

        $parent = $term->parent;

    } elseif($term->term_id > 0) { // Cats with kids

        $parent = $term->term_id;

    } else { // TOP Level Cats

        $parent = 0;

    }   

    $args = array(
        "type" => $post_type,
        "taxonomy" => $taxonomy,
        "parent" => $parent,
        "exclude" => 2,
    );

    $categories = get_categories($args);


    if($categories) {

        echo "<ul>";

        foreach($categories as $cat) {

            echo "<li class='cat-item'>";

                echo "<a href='" . get_term_link($cat) . "'>" , $cat->name , "</a>";

            echo "</li>";

        }

        echo "</ul>";

    }

?>

Answer 1

You could try using the terms_clauses filter

function wpse156370_terms_clauses( $pieces, $taxonomies, $args ) {
    global $wpdb;
    $pieces['fields'] .= $wpdb->prepare( ', (SELECT COUNT(*) FROM '
        . $wpdb->posts . ' p, ' . $wpdb->term_relationships . ' p_tr'
        . ' WHERE p_tr.object_id = p.ID AND p_tr.term_taxonomy_id = tt.term_taxonomy_id'
        . ' AND p.post_status = %s AND p.post_type = %s) AS post_count', 'publish', $args['type'] );
    $pieces['orderby'] = ' HAVING post_count > 0 ' . $pieces['orderby'];
    return $pieces;
}

and then around the get_categories

    add_filter( 'terms_clauses', 'wpse156370_terms_clauses', 10, 3 );
    $categories = get_categories( $args );
    remove_filter( 'terms_clauses', 'wpse156370_terms_clauses', 10, 3 );

It's a hack, misusing HAVING (and $pieces['orderby']) as can't refer to the post_count alias in the WHERE, but ... seems to work.

To get the term links to differentiate add a query var to each, eg

echo "<a href='" . add_query_arg( 'post_type', $post_type, get_term_link($cat) ) . "'>" , $cat->name , "</a>";

And then check for this in a pre_get_posts action

function wpse156370_pre_get_posts($query) {
    $taxonomy = 'art_categories';
    $post_types = array( 'artists', 'educations' );
    if ($query->is_main_query() && $query->is_tax( $taxonomy ) && ( $post_type = get_query_var( 'post_type' ) ) && in_array( $post_type, $post_types ) ) {
        $query->set( 'post_type', $post_type );
    }
}
add_action( 'pre_get_posts', 'wpse156370_pre_get_posts' );