Related posts by category with tag fallback

get-related-posts.php

<?php
/**
 * Get related posts.
 *
 * @param array   $args   WP_Query arguments.
 * @return object         The related posts object.
 * @author Greg Rickaby
 */
function wds_client_get_related_posts( $args = array() ) {

	// Set transient key.
	$transient_key = 'wds_client_related_posts_' . get_the_ID();

	// Attempt to fetch from cache.
	$related_posts = get_transient( $transient_key );

	// If a transient isn't available, run WP_Query...
	if ( false === ( $related_posts ) ) {

		// The default related term type.
		$term_type = 'category';

		// Attempt to get the category.
		$term_ids = wp_get_object_terms( get_the_ID(), 'category', array( 'fields' => 'ids' ) );

		// Fallback to tags if no category is present.
		if ( empty( $term_ids ) ) {
			$term_type = 'tag';
			$term_ids = wp_get_object_terms( get_the_ID(), 'post_tag', array( 'fields' => 'ids' ) );
		}

		// Setup default WP_Query args.
		$defaults = array(
			"{$term_type}__in"       => $term_ids,
			'category__not_in'       => array( 38, 39 ),
			'ignore_sticky_posts'    => 1,
			'no_found_rows'          => true,
			'orderby'                => 'date',
			'order'                  => 'DESC',
			'posts_per_page'         => 3,
			'post__not_in'           => array( get_the_ID() ),
			'update_post_meta_cache' => false,
			'update_post_term_cache' => false,
		);

		// Parse arguments.
		$args = wp_parse_args( $args, $defaults );

		// Run the query!
		$related_posts = new WP_Query( $args );

		// Finally, story the data for 24 hours.
		set_transient( $transient_key, $related_posts, 24 * HOUR_IN_SECONDS );
	}

	return $related_posts;
}