how to best change node type

After playing around with the altertype module I decided to add support for:

1. Gracefull "downgrading" from book page
2. Gracefull "upgrading" to book page

I simply delete or insert a row into the book table when necessary. I'm not a regular PHP or Drupal coder, so feel free to comment/improve.

<?php
// $Id: altertype.module,v 1.2 2005/04/20 15:49:51 kevin Exp $

function altertype_help($section) {
  switch ($section) {
    case 'admin/modules#description':
      return t("Allows editors to change a node's type.");
  }
}

function _altertype_get_node_type($node) {

  if ($node->nid) {

    // The node already exist in the data base.

    $sql = "SELECT type from node WHERE nid=$node->nid";
    $result = db_query($sql);
    $obj =  db_fetch_object($result);
    return $obj->type;

  } else {

    // A new node is born.

    return $node->type;
  }

}

function altertype_nodeapi(&$node, $op, $arg) {

  switch ($op) {

    case 'form post':

      // The node form was just displayed. This hook can be used to
      // add fields at the bottom of the form.
      
      if (user_access('administer nodes')) {
        $options = array();
        foreach (node_list() as $type) {
          $n = new StdClass();
          $n->type = $type;
          $options[$type] = node_invoke($n, 'node_name');
        }
        $output = form_select(t('Node type'), 'altertype', $node->type,
          $options, t("<p>Since different node types have different properties, it is not"
        . " usually a good idea to change the type of a published node.</p>",
        NULL, FALSE, FALSE));

      }
      break;
    case 'validate':
      
      // The user has just finished editing the node and is trying to
      // preview or submit it. This hook can be used to check or even
      // modify the node. Errors should be set with form_set_error().

      if ($node->altertype != NULL
       && $node->altertype != $node->type
       && user_access('administer nodes')) {

      // Added by Karl

      $old_type = _altertype_get_node_type($node);

      $msg = "<B>WARNING:</b> By submitting,  you change the node type from " 
            . "<b>$old_type</b> to <b>$node->altertype</b>.<br/>";
 
     if ($old_type == 'book' and $node->altertype !='book'){
            $msg .= "<br/>Some information about relations bewteen book pages etc may be lost."
                 . " Procede at your own risk.";
      }
 
        $node->type = $node->altertype;   // Orignal node type change by kevin  

      }
      break;

  case 'fields':
  
    // Added by Karl

    // The node is being saved to the database. This hook can be used
    // to declare that additional fields of information should be
    // saved to the node table.

    // Simply changing the $node->type does not suffice if the
    // previous node type was book. After changing node type from book
    // to for example story, the list of child nodes is still rendered
    // at the bottom of the page. After deleting the related
    // rows in the book table it goes away.

    // See discussion at http://drupal.org/node/65118

    $old_type = _altertype_get_node_type($node);

    if ($old_type == 'book' and $node->type !='book'){
      
      // Must clean up the book table

      $msg = "The node type is beeing changed from a book page to a $node->type<br/><br/>";
      
      book_delete($node);
      $msg .= db_affected_rows() . " row(s) deleted from the book table.<br/>";
      
    } else if ($old_type != 'book' and $node->type == 'book') {
      
      // Must create a book entry.

      $msg = "The node type is beeing changed from a $node->type to a book page<br/><br/>";

      book_insert($node);
      $msg .= db_affected_rows() . " row(s) insertet into the book table.<br/>";
      
      $msg .= "<br/><br/><b>NOTE:</b> this is a simple attempt to \"upgrade\" to a book page." 
 . " You have to re-edit the node to set parent and weight.";
    }
    
    break;
  }

  drupal_set_message($msg);      // Added by Karl


  return  $output;
}

?>