(code is in ProductCategory.php, ProductPage.php, and a few others that all extend Page)
$downloadfield = new ComplexTableField(
   $this,
   'RelatedDownloads',
   'DownloadLink',
   array(
      'LinkName' => 'Name'
   ),
   'getCMSFields',
   "DownloadLink.DownloadlinkToPageID = {$this->ID}"
);
$downloadfield->setParentClass('ProductCategory');
$downloadfield->relationAutoSetting = true;

class DownloadLink extends DataObject {
   static $db = array(
      'LinkName' => 'Text'
   );
   static $has_one = array(
      'Document' => 'File',
      'DownloadLinkToPage' => 'Page' // (THIS WORKS AS 'ProductCategory' or 'ProductPage')
   );
   function getCMSFields() {
      $fields = new FieldSet();
      $fields -> push(new TextField("LinkName", "Name"));
      $fields -> push(new ImageField('Document'));
      return $fields;
   }
}