Displaying related posts in a custom post type by a custom taxonomy

Question

I'm trying to display related posts by a custom taxonomy (product_tags) in a custom post type (products). The following code works, but it ignores posts_per_page.

Any ideas?

                global $post;
                $terms = get_the_terms( $post->ID , 'product_tags', 'string');
                $do_not_duplicate[] = $post->ID;

                if(!empty($terms)){
                    echo '<strong>Related Products</strong>';
                    foreach ($terms as $term) {
                        query_posts( array(
                        'post_type' => 'products',
                        'product_tags' => $term->slug,
                        'posts_per_page' => 3,
                        'ignore_sticky_posts' => 1,
                        'orderby' => 'rand',
                        'post__not_in' => $do_not_duplicate) );
                        if(have_posts()) {
                            while ( have_posts() ) : the_post(); $do_not_duplicate[] = $post->ID; ?>
                            <div class="single_related">
                            <?php if (has_post_thumbnail()) { ?>
                                <a href="<?php the_permalink() ?>" title="<?php the_title(); ?>">
                                    <?php the_post_thumbnail( 'related_sm', array('alt' => get_the_title()) ); ?>
                                </a>
                            <?php } else { ?>
                                <a href="<?php the_permalink() ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a>
                            <?php } ?>
                            </div>
                            <?php endwhile; wp_reset_query();
                        }
                    }
                }

Answer 1

If you want to query for multiple taxonomy terms you can use the more advanced, tax_query argument of WP_Query queries. For instance

//Get array of terms
$terms = get_the_terms( $post->ID , 'product_tags', 'string');
//Pluck out the IDs to get an array of IDS
$term_ids = wp_list_pluck($terms,'term_id');

//Query posts with tax_query. Choose in 'IN' if want to query posts with any of the terms
//Chose 'AND' if you want to query for posts with all terms
  $second_query = new WP_Query( array(
      'post_type' => 'products',
      'tax_query' => array(
                    array(
                        'taxonomy' => 'product_tags',
                        'field' => 'id',
                        'terms' => $term_ids,
                        'operator'=> 'IN' //Or 'AND' or 'NOT IN'
                     )),
      'posts_per_page' => 3,
      'ignore_sticky_posts' => 1,
      'orderby' => 'rand',
      'post__not_in'=>array($post->ID)
   ) );

//Loop through posts and display...
    if($second_query->have_posts()) {
     while ($second_query->have_posts() ) : $second_query->the_post(); ?>
      <div class="single_related">
           <?php if (has_post_thumbnail()) { ?>
            <a href="<?php the_permalink() ?>" title="<?php the_title(); ?>"> <?php the_post_thumbnail( 'related_sm', array('alt' => get_the_title()) ); ?> </a>
            <?php } else { ?>
                 <a href="<?php the_permalink() ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a>
            <?php } ?>
       </div>
   <?php endwhile; wp_reset_query();
   }

Note: This seems like it might be a 'secondary loop', i.e. not the 'main' Loop (where posts / a single post are displayed). If so, you should not being use query_posts. See this excellent question.

Answer 2

This probably isn't the correct solution, but I would start by moving $term->slug:

foreach ($terms as $term) {
  $termSlug = $term->slug;
  query_posts( array(
    'post_type' => 'products',
    'product_tags' => $termSlug,
    'posts_per_page' => 3,
    ...