$query
    ->entityCondition('entity_type', 'message')
    ->addTag('no_field_weight');

/**
* Implements hook_query_TAG_alter()
*
* Limits resultset to those courses that do not have a timestamp yet.
*/
function os_classes_query_no_field_weight_alter(QueryAlterableInterface $query) {
  $query->leftJoin('field_data_field_date', 'fd', 'message.mid = fd.entity_id');
  $query->isNull('fd.field_date_value');
}