How to show related posts using custom taxonomy in WordPress

<h3 class="title-projects">projects</h3>

<?php global $post; //query all posts
  $nextTagThumb='-1';
  $yourCategory = '5'; //query category 5 (projects category)
  $saved_ids = array();
  $saved_ids[] = get_the_ID(); //make post ID list of all posts output by the query
  $tags = wp_get_post_tags($post->ID); //get all tags from the current post
  
  // build array of tag ID's
  $tagids = array();
  foreach ($tags as $tag)
  {
    $tagids[] = $tag->term_id;
  }
?> 

<?php
  if ($tags) {
  $what_tag = $tags[($nextTagThumb+'1')]->term_id; //term_id is the tag id
  $args=array(
  'category__in' => array($yourCategory),
  'tag__in' => $tagids,
  'post__not_in' => $saved_ids, 
  'showposts'=>20,
  'caller_get_posts'=>1
  );
  $my_query = new WP_Query($args); //custom query to display all posts that match the $args parameters
  if( $my_query->have_posts() ) {
  while ($my_query->have_posts()) : $my_query->the_post(); 
?>

<li class="post-<?php the_ID(); ?>"> <!-- create list loop of all posts output by the query -->
  <a href="<?php the_permalink() ?>" rel="bookmark" title="Link naar <?php the_title_attribute(); ?>">
    <span class="sidebar-post-title"><?php the_title(); ?></span> <!-- show post title -->
    <span class="sidebar-post-thumb">
    <img src="<?php echo get_post_meta($post->ID, "sidebar-thumb", true);?>" />
    </span> <!-- show post thumbnail -->
  </a> 
</li>

<?php endwhile;
  }
  wp_reset_query();
  $nextTagThumb = ($nextTagThumb+1);
  }
?>

<?php
// Create custom taxonomies for right sidebars
add_action( 'init', 'create_my_taxonomies', 0 );

function create_my_taxonomies() {
	register_taxonomy( 'systems', 'post', array
        ( 'hierarchical' => true, 'label' => 'Systems', 'query_var' => true, 'rewrite' => true ) );
	register_taxonomy( 'applications', 'post', array
       ( 'hierarchical' => true, 'label' => 'Applications', 'query_var' => true, 'rewrite' => true ) );
	register_taxonomy
       ( 'projects', 'post', array( 'hierarchical' => true, 'label' => 'Projects', 'query_var' => true, 'rewrite' => true ) );
} ?>

<h3 class="title-projects">projects</h3>

<?php
global $post;
$terms = get_the_terms( $post->ID , 'projects', 'string');
$do_not_duplicate[] = $post->ID;
 
if(!empty($terms)){
    foreach ($terms as $term) {
        query_posts( array(
        'projects' => $term->slug,
        'category__in' => 5,	// 5 being the projects category	
        'showposts' => 80,
        'caller_get_posts' => 1,
        'post__not_in' => $do_not_duplicate, //exclude the current item
        'orderby'=>title,
        'order'=>ASC
         ) );
         
        if(have_posts()){
            while ( have_posts() ) : the_post(); $do_not_duplicate[] = $post->ID; ?>
                <li class="post-<?php the_ID(); ?>"> <!-- create list loop of all posts output by the query -->
              <a href="<?php the_permalink() ?>" rel="bookmark" title="Link naar <?php the_title_attribute(); ?>">
                <span class="sidebar-post-title"><?php the_title(); ?></span> 
                <!-- show post title, used css to reveal only on hover -->
                <span class="sidebar-post-thumb">
                <img src="<?php echo get_post_meta($post->ID, "sidebar-thumb", true);?>" />
                </span> <!-- show post thumbnail -->
              </a> 
            </li>
            <?php endwhile; wp_reset_query();
        }
    }
}
?>

My name is Boris Hoekmeijer, I'm a webdesigner and graphic designer.
I sometimes run into a problem for which I have to find my own solution because Google just won't tell me. That's when I climb behind my mechanical keyboard, and fire away until a tutorial materializes. One of the great things about the web: we can all help each other!
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Cheers!