Response to https://gist.github.com/1342165

gistfile1.txt

<?php
/**
 * Plugin Name: MMC Parks and Features
 * Description: This plugin demonstrates how to route parent/child URLs like http://example.com/parks/yosemite/half-dome/in WordPress 3.3
 * Author: Mike Schinkel
 * Version: 2
 * Author URL: http://about.me/mikeschinkel
 * Notes:
 *  To answer http://lists.automattic.com/pipermail/wp-hackers/2011-November/041486.html
 *  Assumes a metabox that sets $post->post_parent field for a park feature with it's park's $post->ID.
 *
 */

class MMC_Parks_and_Features {
  static $PARK_FEATURE_URL_REGEX = '^parks/([^/]*)/([^/]*)/?';
  /* MMC DEBUG BEGIN */
  static $FEATURE_DETAIL_URL_REGEX = '^parks/([^/]*)/([^/]*)/([^/]*)/?';
  /* MMC DEBUG END */
  
  static function on_load() {
    add_action( 'init', array( __CLASS__, 'init' ) );
    add_filter( 'request', array( __CLASS__, 'request' ) );
    add_filter( 'post_type_link', array( __CLASS__, 'post_type_link' ), 10, 2 );
    //register_activation_hook( __FILE__, array( __CLASS__, 'activate_deactivate' ) );
    //register_deactivation_hook( __FILE__, array( __CLASS__, 'activate_deactivate' ) );
  } 
  /**
   * Changes the permalink type for any post whose post_type is 'mmc_park_feature'
   *
   * @param string $link
   * @param object $post
   * @return string
   */
  static function post_type_link( $link, $post ) {
    if ( 'mmc_park_feature' == $post->post_type ) {
      $parent = get_post( $post->post_parent );
      $link = preg_replace( '#^(https?://[^/]+/).*$#', "$1parks/{$parent->post_name}/{$post->post_name}/", $link );
    }
	/* MMC DEBUG BEGIN */
    if ( 'mmc_feature_detail' == $post->post_type ) {
      $parent = get_post( $post->post_parent );
      $grandparent = get_post( $parent->post_parent );
      $link = preg_replace( '#^(https?://[^/]+/).*$#', "$1parks/{$grandparent->post_name}/{$parent->post_name}/{$post->post_name}/", $link );
    }
	/* MMC DEBUG END */
    return $link;
  }
  /**
   * Test to make sure the URL is valid per the specified park and park feature referenced in the URL.
   *
   * @param $query_vars
   * @return array
   */
  static function request( $query_vars ) {
    global $wp;
    
    /* MMC DEBUG BEGIN */
    if ( $wp->matched_rule == self::$FEATURE_DETAIL_URL_REGEX ) {
		
      $park_feature = get_page_by_path( $query_vars['mmc_park_feature_qv'], OBJECT, 'mmc_park_feature' );

      $feature_detail_query = new WP_Query( array(
        'name'              => $query_vars['mmc_feature_detail_qv'],
        'post_type'         => 'mmc_feature_detail',
        'fields'            => 'ids',
        'post_parent'       => $park->ID,
        'posts_per_page'    => 1,
        'suppress_filters'  => true,
      ));

      if ( 0 == count( $feature_detail_query->posts ) ) {
        $query_vars = array( 'error' => '404' );
      }    

    }
    /* MMC DEBUG END */
    
    /**
     * Trigger this test if the match URL rewrite rule is the one added by this plugin
     */
    if ( $wp->matched_rule == self::$PARK_FEATURE_URL_REGEX ) {
      /**
       * Lookup the park's post
       */
      $park = get_page_by_path( $query_vars['mmc_park_qv'], OBJECT, 'mmc_park' );
      /**
       * Lookup the park feature's post, scoped to the park
       */
      $park_feature_query = new WP_Query( array(
        'name'              => $query_vars['mmc_park_feature_qv'],
        'post_type'         => 'mmc_park_feature',
        'fields'            => 'ids',
        'post_parent'       => $park->ID,
        'posts_per_page'    => 1,
        'suppress_filters'  => true,
      ));
      /**
       * If the park feature was not found for the park, trigger 404 error.
       */
      if ( 0 == count( $park_feature_query->posts ) ) {
        $query_vars = array( 'error' => '404' );
      }
    }
    return $query_vars;
  }
  /**
   * Register both mmc_park and mmc_park_feature post types.
   *
   * Note: I mostly copied this code from the question, I would typically use a helper function for labels.
   *
   * @return void
   */
  static function init() {
    /**
     * Add the rewrite rule to be checked before other rewrite rules (i.e. 'top')
     */
    add_rewrite_rule(
      self::$PARK_FEATURE_URL_REGEX,
      'index.php?post_type=mmc_park_feature&mmc_park_qv=$matches[1]&mmc_park_feature_qv=$matches[2]',
      'top'
    );
    
	/* MMC DEBUG BEGIN */
    add_rewrite_rule(
      self::$FEATURE_DETAIL_URL_REGEX,
      'index.php?post_type=park_detail&mmc_park_qv=$matches[1]&mmc_park_feature_qv=$matches[2]&mmc_feature_detail_qv=$matches[3]',
      'top'
    );
	/* MMC DEBUG END */

    register_post_type( 'mmc_park', array(
      'labels' => array(
        'name' => _x( 'Parks', 'mmc_park' ),
        'singular_name' => _x( 'Park', 'mmc_park' ),
        'add_new' => _x( 'Add New', 'mmc_park' ),
        'add_new_item' => _x( 'Add New Park', 'mmc_park' ),
        'all_items' => _x( 'Parks', 'mmc_park' ),
        'edit_item' => _x( 'Edit Park', 'mmc_park' ),
        'new_item' => _x( 'New Park', 'mmc_park' ),
        'view_item' => _x( 'View Park', 'mmc_park' ),
        'search_items' => _x( 'Search Parks', 'mmc_park' ),
        'not_found' => _x( 'No Parks found', 'mmc_park' ),
        'not_found_in_trash' => _x( 'No Parks found in Trash', 'mmc_park' ),
        'parent_item_colon' => _x( 'Parent Park', 'mmc_park' ),
        'menu_name' => _x( 'Parks', 'mmc_park' ),
      ),
      'hierarchical' => true,
      'description' => '',
      'supports' => array( 'title', 'editor', 'author', 'revisions',
      'custom-fields', 'comments', 'thumbnail' ),
      'public' => true,
      'show_ui' => true,
      'show_in_nav_menus' => true,
      'publicly_queryable' => true,
      'exclude_from_search' => false,
      'has_archive' => true,
      'query_var' =>  'mmc_park_qv',
      'can_export' => true,
      'rewrite' => array( 'slug' => 'parks', 'with_front' => false ),
      'capability_type' => 'page'
    ));
    
 
    register_post_type( 'mmc_park_feature', array(
      'labels' => array(
        'name' => _x( 'Features', 'mmc_park_feature' ),
        'singular_name' => _x( 'Feature', 'mmc_park_feature' ),
        'add_new' => _x( 'Add New', 'mmc_park_feature' ),
        'add_new_item' => _x( 'Add New Feature', 'mmc_park_feature' ),
        'all_items' => _x( 'Features', 'mmc_park_feature' ),
        'edit_item' => _x( 'Edit Feature', 'mmc_park_feature' ),
        'new_item' => _x( 'New Feature', 'mmc_park_feature' ),
        'view_item' => _x( 'View Feature', 'mmc_park_feature' ),
        'search_items' => _x( 'Search Features', 'mmc_park_feature' ),
        'not_found' => _x( 'No Features found', 'mmc_park_feature' ),
        'not_found_in_trash' => _x( 'No Features found in Trash', 'mmc_park_feature' ),
        'parent_item_colon' => _x( 'Parent Feature', 'mmc_park_feature' ),
        'menu_name' => _x( 'Features', 'mmc_park_feature' ),
      ),
      'hierarchical' => true,
      'supports' => array( 'title', 'editor', 'author', 'revisions', 'custom-fields', 'comments', 'thumbnail' ),
      'public' => true,
      'show_ui' => true,
      'show_in_nav_menus' => true,
      'publicly_queryable' => true,
      'exclude_from_search' => false,
      'has_archive' => true,
      'query_var' => 'mmc_park_feature_qv',
      'can_export' => true,
      'rewrite' => array( 'slug' => 'features', 'with_front' => false ),
      'capability_type' => 'page'
    ));




    register_post_type( 'mmc_feature_detail', array(
      'labels' => array(
        'name' => _x( 'Details', 'mmc_feature_detail' ),
        'singular_name' => _x( 'Detail', 'mmc_feature_detail' ),
        'add_new' => _x( 'Add New', 'mmc_feature_detail' ),
        'add_new_item' => _x( 'Add New Detail', 'mmc_feature_detail' ),
        'all_items' => _x( 'Details', 'mmc_feature_detail' ),
        'edit_item' => _x( 'Edit Detail', 'mmc_feature_detail' ),
        'new_item' => _x( 'New Detail', 'mmc_feature_detail' ),
        'view_item' => _x( 'View Detail', 'mmc_feature_detail' ),
        'search_items' => _x( 'Search Details', 'mmc_feature_detail' ),
        'not_found' => _x( 'No Details found', 'mmc_feature_detail' ),
        'not_found_in_trash' => _x( 'No Details found in Trash', 'mmc_feature_detail' ),
        'parent_item_colon' => _x( 'Parent Detail', 'mmc_feature_detail' ),
        'menu_name' => _x( 'Details', 'mmc_feature_detail' ),
      ),
      'hierarchical' => true,
      'supports' => array( 'title', 'editor', 'author', 'revisions', 'custom-fields', 'comments', 'thumbnail' ),
      'public' => true,
      'show_ui' => true,
      'show_in_nav_menus' => true,
      'publicly_queryable' => true,
      'exclude_from_search' => false,
      'has_archive' => true,
      'query_var' => 'mmc_feature_detail_qv',
      'can_export' => true,
      'rewrite' => array( 'slug' => 'details', 'with_front' => false ),
      'capability_type' => 'page'
    ));
    
  }
}
MMC_Parks_and_Features::on_load();

add_action( 'load-post.php',     'metabox_create' );
add_action( 'load-post-new.php', 'metabox_create' );
add_action( 'save_post',         'metabox_save'   );

function metabox_create() {

	add_meta_box( 'mmc_post_parent', 'Parent Park', 'metabox_display', 'mmc_park_feature', 'side', 'low', array( 'parent_type' => 'mmc_park' ) );
	add_meta_box( 'mmc_post_parent', 'Parent Feature', 'metabox_display', 'mmc_feature_detail', 'side', 'low', array( 'parent_type' => 'mmc_park_feature' ) );

}

function metabox_display( $post, $metabox ) {

	global $wpdb;

	$parent_type = $metabox['args']['parent_type'];
	$query = "SELECT ID, post_title
			  FROM $wpdb->posts
			  WHERE post_type = '{$parent_type}'
			  AND post_status = 'publish'
			  ORDER BY post_title";
	$results = $wpdb->get_results( $query, OBJECT );

	wp_nonce_field( plugin_basename( __FILE__ ), 'mmc_post_parent_noncename' );
	?>
	<select name="mmc_post_parent" id="mmc_post_parent">
		<option value="0"></option>
		<?php
		$parent_post = $post->post_parent;

		foreach ( $results as $result ) {
			if ( $parent_post == $result->ID ) {
				echo '<option value="'. $result->ID . '" selected="selected">' . $result->post_title . '</option>';
			} else {
				echo '<option value="'. $result->ID . '">' . $result->post_title . '</option>';
			}
		}
		?>
	</select>
	<?php

}

function metabox_save( $post_id ) {

	global $post;
	global $wpdb;

	if ( ! wp_verify_nonce( $_POST['mmc_post_parent_noncename'], plugin_basename(__FILE__) ) ) {
		return $post_id;
	}
	
	$data = $_POST['mmc_post_parent'];
	$wpdb->update( $wpdb->posts, array ( 'post_parent' => $data ), array ( 'ID' => $post_id ) );

}