// Check category or query
    $v = new sfValidatorCallback(array(
      'callback' => array($this, 'checkCategoryOrQuery')
    ,) ,
    array(
      'invalid'  => 'Veuillez choisir une rubrique ou un mot clé'
    ));
 
    $this->validatorSchema->setPostValidator($v);

et le callback qui va bien:

  /**
   * Check if a category or a keyword has been set.
   *
   * @param sfValidator $validator
   * @param Array $value Form values
   * @param Array $arguments  Validation arguments
   * 
   * @return mixed
   */
  public function checkCategoryOrQuery($validator, $value, $arguments)
  {
    if (empty($value['category_id']) && empty($value['nature_id']))
    {
      throw new sfValidatorError($validator, 'invalid');
    }
 
    return $value;
  }

  /**
   * Generate the javascript that will initialialize the date picker.
   *
   * @return String
   */
  protected function jqGetJavascript($name)
  {
    sfLoader::loadHelpers('Javascript');
    $main_name_id = $this->generateId($name);
 
    $js = "
jQuery(document).ready(function() {
  $('#%s_%s').datepicker({
      %s %s 
      beforeShow: function() { readLinked('%s', '%s'); },
      onSelect: function() { updateLinked(jQuery('#%s_%s').val(), '%s'); },
      dateFormat: '%s'
  });
  $('#%s_month, #%s_year').change( function() {
    checkLinkedDays('%s');
  });
  checkLinkedDays('%s');
});      
";      
 
    $js = sprintf($js,
      $main_name_id, 
      self::JQ_DEFAULT_DATEPICKUP_ID,
      $this->getOption('minDate'),  
      $this->getOption('maxDate'),  
      $main_name_id,
      self::JQ_DEFAULT_DATEPICKUP_ID, 
      $main_name_id, 
      self::JQ_DEFAULT_DATEPICKUP_ID,
      $main_name_id, 
      $this->jqGetTranslatedDateFormat(),
      $main_name_id, 
      $main_name_id,
      $main_name_id,
      $main_name_id
    );    
 
    return javascript_tag($js); 
  }