How to get terms get_terms() that have posts with a specific post_type?

In WordPress, to get terms using get_terms() and filter them by post type (post_type), you need to perform an additional SQL query since get_terms() does not provide direct options for filtering by post type.

You can either use meta_query or work through relationships with posts. Here’s a step-by-step method:

Example of getting terms that have posts of a specific post_type

1. Step 1: Get the post IDs of the specific post type First, get the IDs of the posts related to the specific type.
2. Step 2: Get terms associated with these posts After retrieving the post IDs, you can filter the terms.

Here’s an example code:

function get_terms_with_post_type($taxonomy, $post_type) {
    global $wpdb;

    // Query to get terms associated with posts of a specific type
    $query = $wpdb->prepare("
        SELECT t.term_id 
        FROM {$wpdb->terms} AS t
        INNER JOIN {$wpdb->term_taxonomy} AS tt ON t.term_id = tt.term_id
        INNER JOIN {$wpdb->term_relationships} AS tr ON tt.term_taxonomy_id = tr.term_taxonomy_id
        INNER JOIN {$wpdb->posts} AS p ON p.ID = tr.object_id
        WHERE tt.taxonomy = %s
        AND p.post_type = %s
        AND p.post_status = 'publish'
        GROUP BY t.term_id
    ", $taxonomy, $post_type);

    // Get term IDs
    $term_ids = $wpdb->get_col($query);

    // Get term objects
    if (!empty($term_ids)) {
        return get_terms([
            'taxonomy' => $taxonomy,
            'include' => $term_ids,
        ]);
    }

    return [];
}

// Usage
$terms = get_terms_with_post_type('category', 'custom_post_type');

foreach ($terms as $term) {
    echo $term->name . '<br>';
}

Explanation of the code:

1. $wpdb->prepare: Used to safely execute SQL queries with parameters.
2. SQL Query:
   • Joins the terms, term_taxonomy, term_relationships, and posts tables.
   • Filters posts by status (publish) and post_type.
   • Groups the results by term IDs to avoid duplicates.
3. get_terms(): Retrieves terms by the list of IDs found in the SQL query.

Alternative method: WP_Query + get_terms

If SQL queries are undesirable, you can use WP_Query to filter posts by post_type and then get terms from the posts. This method is less efficient:

function get_terms_with_wp_query($taxonomy, $post_type) {
    $query = new WP_Query([
        'post_type' => $post_type,
        'post_status' => 'publish',
        'posts_per_page' => -1,
        'fields' => 'ids', // Return only post IDs
    ]);

    if ($query->have_posts()) {
        $post_ids = $query->posts;

        return wp_get_object_terms($post_ids, $taxonomy);
    }

    return [];
}

// Usage
$terms = get_terms_with_wp_query('category', 'custom_post_type');
foreach ($terms as $term) {
    echo $term->name . '<br>';
}

This method does not require direct SQL queries, but it may be slower if you have many posts.

Both approaches will give you the terms related to posts of a specific type.