related-posts-by-term.php

<?php

/**
 * Return related post results by Taxonomy
 * @since  0.1.0
 * @param  string  $taxonomy Taxonomy slug, example: 'category', 'post_tag', 'taxonomy_slug'
 * @param  int     $id       Post ID
 * @return object            Related Query
 */
function wds_get_related( $taxonomy = 'post_tag', $id = 0 ) {

	// Check if an ID was given to check against vs current loop ID
	// 0 is effectively false, so will default to loop's ID
	$id = $id ? absint( $id ) : get_the_ID();
	// Create a transient key out of our taxonomy slug and post ID
	$trans_id = 'wds_related_'. $taxonomy .'-'. $id;

	// Check if a transient refresh is requested
	// (via a query arg added to url, '?delete-trans=true' )
	$reset = isset( $_GET['delete-trans'] ) && $_GET['delete-trans'] == true;
	
	// Check if we have a related query saved to a transient
	if ( false !== ( $trans = get_transient( $trans_id ) ) && ! $reset )
		// if so, and we don't want to refresh the transient, return it
		return $trans;

	// Ok, get our taxomony terms on the post
	$terms = get_the_terms( $id, $taxonomy );

	// If we don't have any terms, return false
	if ( ! isset( $terms[0]->slug ) )
		return false;

	// Get our related query
	$related = new WP_Query( array(
		'posts_per_page' => 8,
		// exclude our current post
		'post__not_in' => array( $id ),
		'tax_query' => array(
			array(
				'taxonomy' => $taxonomy,
				'field' => 'slug',
				// More restrictive
				// 'terms' => wp_list_pluck( $terms, 'slug' ),
				/**
				 * wp_list_pluck serves the same purpose as:
				 * 
				 * $categories = get_the_category( get_the_ID() );
				 * $category_ids = array();
				 * foreach( $categories as $single_category ) $category_ids[] = $single_category->term_id;
				 */

				'terms' => $terms[0]->slug,
			),
		)
	) );
	if ( $related->have_posts() ) {
		// if we have posts, save the query to a transient
		set_transient( $trans_id, $related, 60*60*24 /* a day */ );
		// & return the query
		return $related;
	}

	// Nothing to see here :(
	return false;
}