Not-exists filter for m-n relation in symfony 1.4

I have a many-to-many relation in symfony 1.4. I am using the Doctrine admin generator, and would like to add an "Is Empty" checkbox to the standard sfWidgetFormDoctrineChoice for that field, to do a "WHERE NOT EXISTS" query and find untagged entries. Anyone seen a solution for this?

Thank you!

widget:

class ynWidgetFormDoctrineChoiceEmpty extends sfWidgetFormDoctrineChoice
{
  protected function configure( $options = array(), $attributes = array() )
  {
    parent::configure( $options, $attributes );

    $this->addOption( 'with_empty', false );
    $this->addOption( 'empty_label', 'none' );
    $this->addOption( 'template', '%select%<div>%empty_checkbox% %empty_label%</div>' );
  }

  public function render( $name, $value = null, $attributes = array(), $errors = array() )
  {
    $select = parent::render( $name, $value, $attributes, $errors );

    if ( $this->getOption('with_empty') ) {
      $values = array_merge(array('text' => '', 'is_empty' => false), is_array($value) ? $value : array());

      return strtr($this->getOption('template'), array(
        '%select%'         => $select,
        '%empty_checkbox%' => $this->renderTag('input', array('type' => 'checkbox', 'name' => $name.'[is_empty]', 'checked' => $values['is_empty'] ? 'checked' : '')),
        '%empty_label%'    => $this->renderContentTag('label', $this->translate($this->getOption('empty_label')), array('for' => $this->generateId($name.'[is_empty]'))),
      ));
    }
    else {
      return $select;
    }
  }
}

validator:

class ynValidatorDoctrineChoiceEmpty extends sfValidatorDoctrineChoice
{
  public function configure( $options = array(), $messages = array() )
  {
    parent::configure( $options = array(), $messages = array() );

    $this->addMessage('empty_or_not', 'Select either choices or none');
  }

  protected function doClean( $values )
  {
    if (
      isset( $values['is_empty'] )
      && $values['is_empty'] == 'on'
    ) {
      if ( count( $values ) > 1 ) {
        // 'empty' selected alongside subjects
        throw new sfValidatorError( $this, 'empty_or_not' );
      }

      return $values;
    }
    else {
      return parent::doClean( $values );
    }
  }
}

in FooFormFilter:

public function addBarListColumnQuery( Doctrine_Query $query, $field, $values )
{
  if (!is_array($values))
  {
    $values = array($values);
  }

  if (!count($values))
  {
    return;
  }

  if (
    isset( $values['is_empty'] )
    && $values['is_empty'] == 'on'
  ) {
    $query
      ->andWhere('NOT EXISTS (SELECT fb.bar_id '
        . 'FROM FooBar fb where ' . $query->getRootAlias()
        . '.id=fb.foo_id)');
  }
  else {
    $query
      ->leftJoin($query->getRootAlias().'.FooBar FooBar')
      ->andWhereIn('FooBar.bar_id', $values)
    ;
  }
}