This plugin features a smarty-pants set of functions to determine what content is related to what.

Related WordPress Posts

<?php
/*
Plugin Name:Russell's Related Posts
Description: A smarty-pants set of functions to determine what content is related to what
Version: 0.1
Author: Russell Heimlich
Author URI: http://www.russellheimlich.com
*/

function get_rh_related_data( $args, $post_id = '' ) {
	global $post, $wpdb;
	$post_id = intval( $post_id );
	if( !$post_id && $post->ID ) {
		$post_id = $post->ID;
	}
	
	if( !$post_id ) {
		return false;
	}
	
	$defaults = array(
		'taxonomy' => 'topics',
		'post_type' => array('post'),
		'max' => 5,
	);
	$options = wp_parse_args( $args, $defaults );
	
	$transient_name = 'rh-related-' . $options['taxonomy'] . '-' . $post_id;
	$transient_name = apply_filters( 'rh_related_transient_name', $transient_name, $post_id, $options );
	
	if( isset( $_GET['flush-related-links'] ) && is_user_logged_in() ) {
		echo '<p>Related links flushed! (' . $transient_name . ')</p>';
		delete_transient( $transient_name );
	}
	
	$output = get_transient( $transient_name );
	if( $output !== false && !is_preview() ) {
		//echo $transient_name . ' read!';
		return $output;
	} 
	
	$args = array(
		'fields' => 'ids',
		'orderby' => 'count',
		'order' => 'ASC',
	);
	$orig_terms_set = wp_get_object_terms( $post_id, $options['taxonomy'], $args );
	
	// Make sure each returned term id is an integer.
	$orig_terms_set = array_map( 'intval', $orig_terms_set );
	
	// Store a copy that we'll be reducing by one item for each iteration. 
	$terms_to_iterate = $orig_terms_set;
	
	$post_args = array(
		'fields' => 'ids',
		'post_type' => $options['post_type'],
		'post__not_in' => array( $post_id ),
		'posts_per_page' => 50,

		// For Performance. See https://10up.github.io/Engineering-Best-Practices/php/
		'no_found_rows' => true,
		'update_post_meta_cache' => false,
		
	);
	$output = array();
	while( count( $terms_to_iterate ) > 1 ) {
		
		$post_args['tax_query'] = array(
			array(
				'taxonomy' => $options['taxonomy'],
				'field' => 'id',
				'terms' => $terms_to_iterate,
				'operator' => 'AND',
			)
		);
		
		$posts = get_posts( $post_args );
		
		foreach( $posts as $id ) {
			$id = intval( $id );
			if( !in_array( $id, $output) ) {
				$output[] = $id;
			}
		}
		array_pop( $terms_to_iterate );
	}
	
	$post_args['posts_per_page'] = 10;
	$post_args['tax_query'] = array(
		array(
			'taxonomy' => $options['taxonomy'],
			'field' => 'id',
			'terms' => $orig_terms_set,
		)
	);
	
	$posts = get_posts( $post_args );
		
	foreach( $posts as $count => $id ) {
		$id = intval( $id );
		if( !in_array( $id, $output) ) {
			$output[] = $id;
		}
		if( count( $output ) > $options['max'] ) {
			// We have enough related post IDs now, stop the loop.
			break;
		}
	}
	$output = apply_filters( 'rh_related_data', $output, $options );
	
	if( !is_preview() ) {
		//echo $transient_name . ' set!';
		set_transient( $transient_name, $output, 24 * HOUR_IN_SECONDS );
	}
	
	return $output;
}

function rh_related( $args = array(), $post_id = '' ) {
	$post_ids = get_rh_related_data( $args, $post_id );
	
	if( !$post_ids ) {
		return false;
	}
	
	$defaults = array(
		'post__in' => $post_ids,
		'orderby' => 'post__in',
		'post_type' => array('post'),
		'posts_per_page' => min( array(count($post_ids), 10)),
		'related_title' => 'Related Posts',
		'category__in' => '',  // Must be an ID
		'category__not_in' => '', // Must be an ID
	);
	$options = wp_parse_args( $args, $defaults );
	
	$related_posts = new WP_Query( $options );
	if( $related_posts->have_posts() ):
	?>
	<div class="widget">
		<h2 class="header"><?php echo $options['related_title']?></h2>
		<div class="inner with-posts">
			<?php while ( $related_posts->have_posts() ):
				$related_posts->the_post();
			?>
				<a class="post" href="<?php the_permalink();?>">
					<div class="meta">
						<?php do_action( 'rh_related_before_date', $related_posts ); ?>
						<span class="date"><?php the_time('M j, Y'); ?></span>
						<?php do_action( 'rh_related_after_date', $related_posts ); ?>
					</div>
					<h2><?php the_title();?></h2>
				</a>
			<?php endwhile;
				wp_reset_postdata();
			 ?> 
		</div>
	</div>
	<?php
	endif;
	
}