drupal7 addExpression+union+分页

global $user;
  $query_single = db_select('mp_order_singlepay', 'ms')
      ->fields('ms', array('order_id','order_time', 'article_name','total_amount','status','payment_method'));
  $query_single->condition('isdelete', 0);
  $query_single->condition('purchaser_id', $user->uid);
  $query_single->addExpression('0', 'is_single');//重点 等于增加了一列

  $query_month = db_select('mp_order_monthspay', 'mm')
      ->fields('mm', array('order_id','order_time', 'product_name','total_amount','status','payment_method'));
  $query_month->condition('isdelete', 0);
  $query_month->condition('purchaser_id', $user->uid);
  $query_month->addExpression('1', 'is_single');//重点

$query = Database::getConnection() ->select($query_single->union($query_month)) 
->fields(NULL, array('order_id','order_time', 'article_name','total_amount','status','payment_method','t')) //fields里要取出增加的自定义列名
->orderBy('order_time','DESC'); 

$total_rows=$query->countQuery()->execute()->fetchField(); 

$per_pager=5;//每页行数 

$objects= $query->extend('PagerDefault')-> limit($per_pager)-> execute() -> fetchAll(); 

var_dump($objects);