get_categories for custom post type with a specific custom taxonomy attached

Hi @daveaspi:

What you want to do is common but not well handled in WordPress core. There are probably ways to do it without custom SQL but I don”t think they would scale for a large number of posts. Below is a function I wrote called get_cross_referenced_terms() that will get what you want, complete with an example how to use it.

This following code can be placed in the root of your WordPress site in a test.php file to
see it work. You can then copy the function get_cross_referenced_terms() into your theme’s functions.php file or into a .php file of a plugin you might be working on:

<?php 

  include('wp-load.php');

  $nike = get_term_by('slug','nike','brand'); // This here just to illustrate

  $terms = get_cross_referenced_terms(array(
    'post_type'        => 'product',
    'related_taxonomy' => 'brand',
    'term_id'          => $nike->term_id,
  ));
  foreach($terms as $term) {
    echo "<p>{$term->name}</p>";
  }

function get_cross_referenced_terms($args) {
  global $wpdb;
  $args = wp_parse_args($args,array(
    'post_type'        => 'post',
    'taxonomy'         => 'category',
    'related_taxonomy' => 'post_tag',
    'term_id'          => 0,
  ));
  extract($args);
  $sql = <<<SQL
SELECT DISTINCT
  {$wpdb->terms}.*,
  COUNT(*) AS post_count
FROM
  {$wpdb->terms}
  INNER JOIN {$wpdb->term_taxonomy} ON {$wpdb->terms}.term_id={$wpdb->term_taxonomy}.term_id
  INNER JOIN {$wpdb->term_relationships} ON {$wpdb->term_taxonomy}.term_taxonomy_id={$wpdb->term_relationships}.term_taxonomy_id
  INNER JOIN {$wpdb->posts} ON {$wpdb->term_relationships}.object_id={$wpdb->posts}.ID
  INNER JOIN {$wpdb->term_relationships} related_relationship ON {$wpdb->posts}.ID=related_relationship.object_id
  INNER JOIN {$wpdb->term_taxonomy} related_term_taxonomy ON related_relationship.term_taxonomy_id=related_term_taxonomy.term_taxonomy_id
  INNER JOIN {$wpdb->terms} related_terms ON related_term_taxonomy.term_id=related_terms.term_id
WHERE 1=1
  AND (related_term_taxonomy.taxonomy<>{$wpdb->term_taxonomy}.taxonomy OR related_terms.term_id<>{$wpdb->terms}.term_id)
  AND {$wpdb->posts}.post_type="%s"
  AND {$wpdb->term_taxonomy}.taxonomy='%s'
  AND related_term_taxonomy.taxonomy='%s'
  AND related_terms.term_id=%d
GROUP BY
  {$wpdb->terms}.term_id
SQL;
  $sql = $wpdb->prepare($sql,$post_type,$taxonomy,$related_taxonomy,$term_id);
  $terms = $wpdb->get_results($sql);
  return $terms;
}