Permalinks in Custom Post types

Take a look at my answer to the following question. You can modify my parse_request() to get what you need:

• “Remove taxonomy slug from a custom hierarchical taxonomy permalink”.

And you can read the following to see why what you want can cause issues in WordPress and requires a complex solution:

• “Category in Permalinks Considered Harmful”

UPDATE

Based on the comment I decided to go ahead and implement this as the next revision of the plugin. It’s really only tested for the use-case for this question and the one from the prior referenced question. Over time I plan to implement it for all potential use-cases, as questions or clients occur to drive the demand.

Here is the code you’d place in your theme’s functions.php file to set it up for the post_type 'custom':

add_action('init','init_url_routes');
function init_url_routes() {
  $post_type="custom"; // Change this to your actual post_type name
  register_url_route(array('post_type'=>$post_type));
}

And here is the plugin which you can place in /wp-content/mu-plugins/:

<?php
/*
Filename: wp-extended.php
Plugin Name: WP Extended for Taxonomy URL Routes
Author: Mike Schinkel
Version: 0.2
*/
function register_url_route($args=array()) {
  WP_Extended::register_url_route($args);
}
class WP_Extended extends WP {
  static $root = array();
  static function on_load() {
    add_action('setup_theme',array(__CLASS__,'setup_theme'));
  }
  static function register_url_route($args) {
    if (isset($args['taxonomy']))
      self::$root['taxonomy'][$args['taxonomy']] = get_taxonomy($args['taxonomy']);
    if (isset($args['post_type']))
      self::$root['posts'][$args['post_type']] = get_post_type_object($args['post_type']);
  }
  static function setup_theme() { // Setup theme is 1st code run after WP is created.
    global $wp;
    $wp = new WP_Extended();  // Replace the global $wp
  }
  function parse_request($extra_query_vars="") {
    $path = $_SERVER['REQUEST_URI'];
    $domain = str_replace('.','\.',$_SERVER['SERVER_NAME']);
    $root_path = preg_replace("#^https?://{$domain}(/.*)$#",'$1',WP_SITEURL);
    if (substr($path,0,strlen($root_path))==$root_path)
      $path = substr($path,strlen($root_path));
    list($path) = explode('?',$path);
    $path_segments = explode("https://wordpress.stackexchange.com/",trim($path,"https://wordpress.stackexchange.com/"));
    // This does not handle ordering priority of type to match yet
    $matched = $this->parse_post_type_request($path_segments);
    if (!$matched)
      $matched = $this->parse_taxonomy_request($path_segments);
    if ($matched) {
      // This is hamfisted but necessary in some cases.
      // TODO: Look into ways to have more finesse with this.
      remove_action('template_redirect','redirect_canonical');
    } else {
      parent::parse_request($extra_query_vars); // Delegate to WP class
    }
  }
  function parse_post_type_request($path_segments) {
    // This does not handle heirarchical pages yet
    $post_id = false;
    global $wpdb;
    $sql =<<<SQL
SELECT
  ID
FROM
  {$wpdb->posts}
WHERE 1=1
  AND post_status="publish"
  AND post_type="%s"
  AND post_name="%s"
SQL;
    if (is_array(self::$root['posts'])) {
      foreach(self::$root['posts'] as $post_type => $post_type_object) {
        $sql = $wpdb->prepare($sql,$post_type,$path_segments[0]);
        $post_id = $wpdb->get_var($sql);
        if ($post_id) {
          $this->query_vars[($post_type=='page' ? 'page_id' : 'p')] = $post_id;
          unset($path_segments[0]);  // Remove from future consideration
          break;
        }
      }
    }
    return ($post_id);
  }
  function parse_taxonomy_request($path_segments) {
    $taxonomy_term = array();
    $parent_id = 0;
    if (is_array(self::$root['taxonomy'])) {
      foreach(self::$root['taxonomy'] as $taxonomy_slug => $taxonomy) {
        $terms = get_terms($taxonomy_slug);
        foreach($path_segments as $segment_index => $path_segment) {
          foreach($terms as $term_index => $term) {
            if ($term->slug==$path_segment) {
              if ($term->parent!=$parent_id) { // Make sure we test parents
                $taxonomy_term = array();
              } else {
                $parent_id = $term->term_id; // Capture parent ID for verification
                $taxonomy_term[] = $term->slug; // Collect slug as path segment
                unset($terms[$term_index]); // No need to scan it again
              }
              unset($path_segments[$segment_index]);  // Remove from future consideration
              break;
            }
          }
        }
        if (count($taxonomy_term))
          break;
      }
      if (count($taxonomy_term)) {
        $path = implode("https://wordpress.stackexchange.com/",$taxonomy_term);
        switch ($taxonomy_slug) {
          case 'category':
            $this->query_vars['category_name'] = $path;
            break;
          case 'post_tag':
            $this->query_vars['tag'] = $path;
            break;
          default:
            $this->query_vars['taxonomy'] = $taxonomy_slug;
            $this->query_vars['term'] = $path;
            break;
        }
      }
    }
    return count($taxonomy_term);
  }
}
WP_Extended::on_load();