【WordPress】記事詳細ページに関連記事一覧のリンクを設置

<?php
  $categories = get_the_category();
  $category_ID = array();
  foreach($categories as $category):
    array_push( $category_ID, $category -> cat_ID);
  endforeach ;
  $args = array(
    'post__not_in' => array($post -> ID),
    'posts_per_page'=> 3,
    'category__in' => $category_ID,
    'orderby' => 'rand',
  );
  $query = new WP_Query($args); 
?>

<?php if( $query -> have_posts() ): while ($query -> have_posts()): $query -> the_post(); ?>
  <a href="<?php the_permalink() ?>">
    <?php the_title(); ?>
  </a>
<?php endwhile; else:?>
  <p>関連記事はありません</p>
<?php endif; ?>
<?php wp_reset_postdata(); ?>