WordPress Related Posts Tutorial, Part 1

related-posts-01.php

<?php

/* 
 * NOTE: This is a portion of a tutorial:
 *
 * @link https://johnregan3.wordpress.com/how-to-get-wordpress-related-posts
 */
function jr3_get_related_posts( $post_id = 0, $args = array(), $count = 3 ) {
	// Prep the output.
	$posts = array();
	
	// Make sure we have a good post_id.  If not, grab the current post.
	if ( empty( $post_id ) || ! is_numeric( $post_id ) ) {
		$post_id = get_the_ID();
	}

	// If we still don't have a good ID, bail.
	if ( empty( $post_id ) ) {
		return array();
	}
  
	/*
	 * Start by filtering the input $args.
	 * wp_parse_args() allows us to set default arguments
	 * that can be overriden by input.
	 * 
	 * Note that we get the post type of the requested post,
	 * and we set the author of the current post.
	 */
	$args = wp_parse_args( $args, array(
		'posts_per_page' => 20, // For sanity.
		'orderby'        => 'title',
		'post_type'      => get_post_type( $post_id ),
		'post_status'    => 'publish',
		'author'         => get_post_field( 'post_author', $post_id ),
		'no_found_rows'  => true, // Speeds up query.
	) );
  
	/* 
	 * Set up taxonomy queries.
	 * This will get us related posts on the same topic(s).
	 */
	$post_obj = get_post( $post_id );
  
	// Get all taxonomies (e.g., "Tags," "Categories") that this post has.
	$taxonomies = get_object_taxonomies( $post_obj, 'names' );

	/*
	 * Sanity check.
	 * Limit the number of taxonomies to prevent getting creating a heavy query.
	 */
	$taxonomies = array_slice( $taxonomies, 0, 3 );

	/*
	 * Get each term associated with the post and set 
	 * a taxonomy query for it.
	 */
	foreach ( $taxonomies as $taxonomy ) {
    
		// Get each term (e.g., single tag or single category) that this post has.
		$terms = get_the_terms( $post_id, $taxonomy );
		if ( empty( $terms ) ) {
			continue;
		}
    
		// Get the slug of each term in the list.
		$term_list           = wp_list_pluck( $terms, 'slug' );
		$args['tax_query'][] = array(
			'taxonomy' => $taxonomy,
			'field'    => 'slug',
			'terms'    => $term_list,
		);
	}

	/* 
	 * If we have more than one taxonomy, use "OR" 
	 * to basically say the posts can use any of the terms.
	 * 
	 * Otherwise, "AND" will mean we will only look for posts
	 * that contain all of the same terms of the current post, which
	 * can be quite specific and restrictive.
	 */
	if ( isset( $args['tax_query'] ) && count( $args['tax_query'] ) > 1 ) {
		$args['tax_query']['relation'] = 'OR';
	}

	// Make sure the current post is not part of what we get back from the query.
	$args['post__not_in'] = array( $post_id );
  
	// Run the query.
	$query = new WP_Query( $args );

	// Run the query.
	$query = new WP_Query( $args );
	if ( ! empty( $query->posts ) && is_array( $query->posts ) ) {
		$posts = $query->posts;
	}
	
	// Make sure we don't exceed the number of $count.
	return array_slice( $posts, 0, $count );
}