How to get_terms for post type

Ever registered taxonomies for multiple post types in WordPress? Then you already know that the get_terms function has one important drawback: it does not provide post type parameter. It’s impossible to display terms related to selected post type (or post types) or get the correct posts count for those terms.

But we’ve developed a solution, based on WordPress native terms_clauses filter.

// Handle the post_type parameter given in get_terms function
function df_terms_clauses($clauses, $taxonomy, $args) {
	if (!empty($args['post_type']))	{
		global $wpdb;

		$post_types = array();

		foreach($args['post_type'] as $cpt)	{
			$post_types[] = "'".$cpt."'";
		}

	    if(!empty($post_types))	{
			$clauses['fields'] = 'DISTINCT '.str_replace('tt.*', 'tt.term_taxonomy_id, tt.term_id, tt.taxonomy, tt.description, tt.parent', $clauses['fields']).', COUNT(t.term_id) AS count';
			$clauses['join'] .= ' INNER JOIN '.$wpdb->term_relationships.' AS r ON r.term_taxonomy_id = tt.term_taxonomy_id INNER JOIN '.$wpdb->posts.' AS p ON p.ID = r.object_id';
			$clauses['where'] .= ' AND p.post_type IN ('.implode(',', $post_types).')';
			$clauses['orderby'] = 'GROUP BY t.term_id '.$clauses['orderby'];
		}
    }
    return $clauses;
}
add_filter('terms_clauses', 'df_terms_clauses', 10, 3);

To make use of this just add post_type argument to get_terms function like so:

// Get the categories for post and product post types
$categories = get_terms('category', array(
 	'post_type' => array('post', 'product'),
 	'fields' => 'all'
));

You may use any of the built-in arguments with one exception: in order to get the correct posts count you must keep the ‘fields’ set to default ‘all’. If you don’t provide the post_type parameter the get_terms results will not get affected by the term_clauses filter.

Edit:
A new, updated approach – use it if you have issues with the one above:

/**
 * Extend get terms with post type parameter.
 *
 * @global $wpdb
 * @param string $clauses
 * @param string $taxonomy
 * @param array $args
 * @return string
 */
function df_terms_clauses( $clauses, $taxonomy, $args ) {
	if ( isset( $args['post_type'] ) && ! empty( $args['post_type'] ) && $args['fields'] !== 'count' ) {
		global $wpdb;

		$post_types = array();

		if ( is_array( $args['post_type'] ) ) {
			foreach ( $args['post_type'] as $cpt ) {
				$post_types[] = "'" . $cpt . "'";
			}
		} else {
			$post_types[] = "'" . $args['post_type'] . "'";
		}

		if ( ! empty( $post_types ) ) {
			$clauses['fields'] = 'DISTINCT ' . str_replace( 'tt.*', 'tt.term_taxonomy_id, tt.taxonomy, tt.description, tt.parent', $clauses['fields'] ) . ', COUNT(p.post_type) AS count';
			$clauses['join'] .= ' LEFT JOIN ' . $wpdb->term_relationships . ' AS r ON r.term_taxonomy_id = tt.term_taxonomy_id LEFT JOIN ' . $wpdb->posts . ' AS p ON p.ID = r.object_id';
			$clauses['where'] .= ' AND (p.post_type IN (' . implode( ',', $post_types ) . ') OR p.post_type IS NULL)';
			$clauses['orderby'] = 'GROUP BY t.term_id ' . $clauses['orderby'];
		}
	}
	return $clauses;
}

add_filter( 'terms_clauses', 'df_terms_clauses', 10, 3 );