无法更新多个文档

我正在尝试更新符合以下条件的所有文档。\n出于某种原因，控制台出现了一个错误，提示 alerts.save 不是一个函数。\n以下是我更新记录的方法。\n问题是我尝试更新多个文档吗？\n

router.route('/mark_as_read/:ownerId')
.put(function(req, res){
  var alerts = new Alerts();
  Alerts.find({owner_id: req.params.ownerId}, function(err, alerts){
    if(err)
        res.send(err);
    alerts.viewed = true;
    console.log('Alerts: ', alerts);
    alerts.save(function(err){
      if(err)
        res.send(err);
      res.json(alerts)
    });
  });
});

\n这是一个我进行GET请求时返回的数据示例：\n

  { _id: 59aed31e78fbbea611518ddf,
    message: 'Hammer Hammer at hammer@example.com',
    alert_type: 'Share',
    owner_id: '59a6e66d5ab67150837d96a8',
    __v: 0,
    viewed: false,
    date: 2017-09-05T16:38:54.437Z },
  { _id: 59aed6035fee3fa895045693,
    message: 'Paul Pataa at paul@example.com',
    alert_type: 'Share',
    owner_id: '59a6e66d5ab67150837d96a8',
    __v: 0,
    viewed: false,
    date: 2017-09-05T16:51:15.193Z },
  { _id: 59aedba6fec346aafae9121c,
    message: 'Marky Mark at marks@example.com',
    alert_type: 'Share',
    owner_id: '59a6e66d5ab67150837d96a8',
    __v: 0,
    viewed: false,
    date: 2017-09-05T17:15:18.876Z },
  { _id: 59aedc36fec346aafae91228,
    message: 'Lilly Lap at sammy@example.com',
    alert_type: 'Share',
    owner_id: '59a6e66d5ab67150837d96a8',
    __v: 0,
    viewed: false,
    date: 2017-09-05T17:17:42.951Z }

\n我尝试遍历 alerts 数组，但对我来说不起作用：\n

router.route('/mark_as_read/:ownerId')
.put(function(req, res){
  var alerts = new Alerts();
  Alerts.find({owner_id: req.params.ownerId}, function(err, alerts){
    if(err)
        res.send(err);
    for(var i=0; i < alerts.length; i++){
      alerts.viewed = true;
      alerts[i].save(function(err){
        if(err)
          console.log('Error in updating an alert');
      })
    }
  });
});