Plugin demonstrates how to route parent post/child post URLs WordPress 3.3

parks-and-features.php

<?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
 * 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/([^/]*)/([^/]*)/?';

	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, 4 );
	  register_activation_hook( __FILE__, array( __CLASS__, 'activate_activate' ) );
	}
	static function activate_activate() {
		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'
		);
		flush_rewrite_rules();
	}
  /**
   * Changes the permalink type for any post whose post_type is 'mmc_park_feature'
   *
   * @param string $link
   * @param object $post
   * @param bool $leavename
   * @param string $sample
   * @return string
   */
  static function post_type_link( $link, $post, $leavename, $sample ) {
    if ( 'mmc_park_feature' == $post->post_type ) {
      $parent = get_post( $post->post_parent );
      $parent_slug = isset( $parent->post_name ) ? $parent->post_name : '%park%';
      $page_slug = $sample ? '%postname%' : $post->post_name;
      $link = preg_replace( '#^(https?://[^/]+/).*$#', "$1parks/{$parent_slug}/{$page_slug}/", $link );
    }
    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;
    /**
     * 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')
     */
    register_post_type( 'mmc_park', array(
      'labels' => array(
        'name' => _x( 'Parks', 'mmc_pack' ),
        'singular_name' => _x( 'Park', 'mmc_pack' ),
        'add_new' => _x( 'Add New', 'mmc_pack' ),
        'add_new_item' => _x( 'Add New Park', 'mmc_pack' ),
        'all_items' => _x( 'Parks', 'mmc_pack' ),
        'edit_item' => _x( 'Edit Park', 'mmc_pack' ),
        'new_item' => _x( 'New Park', 'mmc_pack' ),
        'view_item' => _x( 'View Park', 'mmc_pack' ),
        'search_items' => _x( 'Search Parks', 'mmc_pack' ),
        'not_found' => _x( 'No Parks found', 'mmc_pack' ),
        'not_found_in_trash' => _x( 'No Parks found in Trash', 'mmc_pack' ),
        'parent_item_colon' => _x( 'Parent Park', 'mmc_pack' ),
        'menu_name' => _x( 'Parks', 'mmc_pack' ),
      ),
      'hierarchical' => true,
      'description' => '',
      'supports' => array( 'title', 'editor', 'author', 'revisions',
      'custom-fields', 'comments', 'thumbnail' ),
      'public' => true,
      'show_ui' => true,
//      'show_in_menu' => 'mmc_menu',
      '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', 'permalink', 'editor', 'author', 'revisions', 'custom-fields', 'comments', 'thumbnail' ),
      'public' => true,
      'show_ui' => true,
//      'show_in_menu' => 'mmc_menu',
      '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'
    ));
  }
}
MMC_Parks_and_Features::on_load();