I created two custom post types and one custom taxonomy. I wanted to be able to link the two different post types by applying the same taxonomy to both. How could I do this?


Custom Post Type One: Places (eg. The Grand Canyon) Custom Post Type Two: Trips (eg. Journey Aboard the Grand Canyon Railway, Grand Canyon National Park South Rim Bus Tour) Custom Taxonomy: Location (eg. Arizona)

Now if I add a Place called Grand Canyon and two Trips Called Journey Aboard... & South Rim Bus... I would tag all with the Location Arizona, because they are all related to Arizona.

Now, how would I be able to automatically display the list of all Trips that are related to a Place based on the Location that they are tagged with?

Also, Is there a better way to accomplish this?

1 Answer 1


Hi @Chris M:

Yes, what you are planning is one of the easier ways to accomplish a relationship between post where the user defines the relationship through a shared value.

There are several ways you could make this work but since WordPress doesn't give you an easy single query way to do it easily I decided to subclass WP_Query named Place_Trip_Query for you so that you can accomplish this with one SQL query. Normally I like to avoid SQL queries but this is an area where I don't think it can be helped.

The class uses the 'posts_where' and 'posts_join' hooks to let us modify the SQL query to associate the Place and Trip posts that have been tagged by the same Location term for a given place name (the query uses a "starts with" query so "Grand" will match "Grand Canyon").

You can place this code in your theme's functions.php file or in a PHP file of a plugin you might be writing:

class Place_Trip_Query extends WP_Query {
  var $place_name = false;
  function __construct($args=array()) {
    if (isset($args['place_name']))
      $this->place_name = $args['place_name'];
  function get_posts($args=array()) {
  static function posts_join($join,$query) {
    global $wpdb;
    $join .= <<<SQL
INNER JOIN {$wpdb->term_relationships} 
  ON {$wpdb->term_relationships}.object_ID={$wpdb->posts}.ID
INNER JOIN {$wpdb->term_taxonomy} 
  ON {$wpdb->term_taxonomy}.term_taxonomy_id={$wpdb->term_relationships}.term_taxonomy_id 
 AND {$wpdb->term_taxonomy}.taxonomy='location'
INNER JOIN {$wpdb->term_relationships} place_relationships 
  ON place_relationships.term_taxonomy_id={$wpdb->term_relationships}.term_taxonomy_id
INNER JOIN {$wpdb->posts} place 
  ON place.ID=place_relationships.object_id
    return $join;
  static function posts_where($where,$query) {
    global $wpdb;
    $sql = <<<SQL
AND place.post_type='place'
AND place.ID<>{$wpdb->posts}.ID
AND place.post_title LIKE '%s'
    // Search for something that starts with "{place_name}"...
    $where .= $wpdb->prepare($sql,$query->place_name.'%'); 
    return $where;

You can use this class just like you might use the WP_Query with these args. Here's a standalone file you could call test.php and drop in the root of your website to test the class and show you how it works:

include '../wp-load.php';
$query = new Place_Trip_Query(array(
  'place_name'=>'Grand Canyon', // Here's where you put your place name
echo "<ul>";
foreach($query->posts as $trip) {
  echo "<li>{$trip->post_title}</li>";
echo "</ul>";

And for others reading this who would like to test this here's some code to create the Places and Trips custom post types and the Location custom taxonomy. You can also place this in your theme's functions.php file:

function places_trips_init() {
      'labels'          => make_post_type_labels('Place'),
      'public'          => true,
      'show_ui'         => true,
      'query_var'       => 'place',
      'rewrite'         => array('slug' => 'places'),
      'hierarchical'    => true,
      //'supports'        => array('title','editor','custom-fields'),
      'labels'          => make_post_type_labels('Trip'),
      'public'          => true,
      'show_ui'         => true,
      'query_var'       => 'trip',
      'rewrite'         => array('slug' => 'trips'),
      'hierarchical'    => true,
      //'supports'        => array('title','editor','custom-fields'),
  register_taxonomy('location', array('place','trip'), array(
    'hierarchical'    => true,
    'labels'          => make_post_type_labels('Location'),
    'query_var'       => 'location',
    'rewrite'         => array('slug' => 'locations' ),
  global $wp_rewrite;
  $wp_rewrite->flush_rules(false);  // This should really be done in a plugin activation
function make_post_type_labels($singular,$plural=false,$args=array()) {
  if ($plural===false)
    $plural = $singular . 's';
  elseif ($plural===true)
    $plural = $singular;
  $defaults = array(
    'name'              =>_x($plural,'post type general name'),
    'singular_name'      =>_x($singular,'post type singular name'),
    'add_new'            =>_x('Add New',$singular),
    'add_new_item'      =>__("Add New $singular"),
    'edit_item'          =>__("Edit $singular"),
    'new_item'          =>__("New $singular"),
    'view_item'          =>__("View $singular"),
    'search_items'      =>__("Search $plural"),
    'not_found'          =>__("No $plural Found"),
    'not_found_in_trash'=>__("No $plural Found in Trash"),
    'parent_item_colon' =>'',
  return wp_parse_args($args,$defaults);
  • I don't see the difference between your code and the built-in get_posts( array( 'post_type' => 'trip', 'taxonomy' => 'location', 'term' => 'grand-canyon' ) ). Can you explain what your code does extra, or why get_posts() would not work?
    – Jan Fabry
    Dec 31, 2010 at 15:35
  • @Jan Fabry - That's what I almost wrote, then I realized what he was asking. Grand Canyon is not a term, it's a custom post type where post_type='place' and it is matched to custom post types of post_type='trip' by both having the Location term 'Arizona'. Does that explain? Dec 31, 2010 at 18:26
  • OK, I assumed this would be used on a single-place.php file, so while displaying the Grand Canyon post. Then you can ask for the location trip term that post is attached to, and use these to do your get_posts query. Of course, I should not have used grand-canyon as the term, but arizona.
    – Jan Fabry
    Jan 1, 2011 at 10:36
  • @Jan Fabry - Yes, I think that could work too albeit with more queries to the database. Jan 1, 2011 at 21:06

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