List non-empty categories from a custom post type

Question

I have a Custom Post Type that uses the default category, the same as the posts, and some other post types in my site. How can I list all non-empty categories from the custom post type?

For instance, if I have categories “Cat 1”, “Cat 2" and “Cat 3” and I have a default post with a category of “Cat 1", and a custom post with categories “Cat 2” and “Cat 3". How would I write a function that would return “Cat 2” and “Cat 3"?

It seems like the get_terms() functions don’t support a post_type argument. Is there a way I can add some other query to a get_terms() function?

I know that it is possible to hide unused terms:

$terms = get_terms( array(
    'taxonomy' => 'category',
    'hide_empty' => true, <-----
) );

However this hides empty for all post types, I'd like to hide empty for just a custom post type.

Answer 1

You can modify the term query to count for a specfic post type.

Set a unique argument parameter to your term query.

$terms = get_terms( array(
    'taxonomy' => 'category',
    'hide_empty' => true,
    'my_param' => 'book', // Name of the custom post type you want to filter.
) );

And then in function.php add this filter.

add_filter('terms_clauses', function($clauses, $taxonomy, $args) {
    global $wpdb;

    $post_type = $args['my_param'] ?? '';

    // Proceed only if your custom parameter is set.
    if (empty($post_type)) {
        return $clauses;
    }

    // Modify the query to only include terms that have posts in the specified post type.
    $clauses['join'] .= " LEFT JOIN {$wpdb->term_relationships} AS tr ON tt.term_taxonomy_id = tr.term_taxonomy_id LEFT JOIN {$wpdb->posts} AS p ON tr.object_id = p.ID AND p.post_type = '{$post_type}'";
    $clauses['where'] .= " GROUP BY t.term_id HAVING COUNT(DISTINCT p.ID) > 0";

    return $clauses;
}, 10, 3);

Answer 2

This will end up being a little convoluted since the taxonomy just describes a relationship that may belong to any of the post types, so there's not a direct data link for what you're trying to fetch.

But you could do something like this (untested), which could be a fairly heavy query if you're doing on every page load or something, so maybe consider running it on a cron job and storing it somewhere or something.

<?php

    $args = array(
        'post_type' => 'my-post-type',
        'posts_per_page' => -1,
    );

    $fetch = get_posts( $args );
    $cat_r = array();

    // Iterate the posts and get the categories
    foreach( $fetch as $item ) {
        $cats = get_the_terms( $item->ID, 'category' );
        $cat_r[] = $cats;
    }

    // Outputs an array of term objects
    echo '<pre>';
    print_r( $cat_r );
    echo '</pre>';