Drupal 5: Adding validation to a node form to ensure its title is unique

Here’s a code snippet that ensures the submitted title is unique across all nodes.

<?php
function MYMODULE_form_alter($form_id, $form) {
  if (substr($form_id, -10)=='_node_form') {
    // add custom form validation function
    $form['#validate'] = array_merge(array('_MYMODULE_helper_validate' => array()), $form['#validate']);
  }
}

function _MYMODULE_helper_validate($form_id, $form_values, $form) {
  // ensure title is unique
  if (strlen($form_values['title'])) {
    // check for unique title
    $sql = "select distinct title from {node} where 1=1 ";
    // don't want to include current title
    if ($form_values['nid']) $sql .= "and nid != '" . db_escape_string($form_values['nid']) . "'";

    $resource = db_query($sql);
    $titles = array();
    while($result = db_fetch_array($resource)) $titles[] = $result['title'];

    if (in_array($form_values['title'],$titles))
      form_set_error('title', $form['title']['#title'] . ' has already been submitted.');
  }
}
?>