Question

我从帖子数据中获取了一系列用户电子邮件。我想找到与每封电子邮件相关的_id。我尝试了这个循环：

var studentIds = [];

for (var i = studentEmails.length - 1; i >= 0; i--) {
  var email = studentEmails[i];

  User.findOne({"email": email}, (err, student) => {
    if (err) {
      console.log("ERROR" + err);
    }
    if (student) {
      var id = student._id;
      studentIds.push(id);
      console.log("STUDENT: " + student);
    }
  });

}

// Outside for loop
console.log('END');

但是，这会记录以下内容：

END
STUDENT: { _id: 5a11e667d7333203337cd9a4,
  name: 'Patrick Jacobs',
  email: 'windvaan@live.nl',
  password: '$2a$10$CiSw/VH1HCaPtW6Sjz0X4.4avVoLsAH6iyF3FhidorahwLt1WDXoC',
  __v: 0 }
STUDENT: { _id: 5a0f7dfb64b5a6000417c662,
  name: 'Carlo Jacobs',
  email: 'carlojacobs91@gmail.com',
  password: '$2a$10$fiIosS4Jo5ehuCp3TfltSOnpypPMWSMvzlb7phRWmNGBtDz5W1rCG',
  __v: 0 }

如您所见，首先打印END。我不希望这样。我假设for循环是异步的？我怎样才能使它同步？

提前谢谢！

Answer 1

这不是for循环，而是异步的User.findOne调用。

关注this answer，您可以使用streamline.js;尝试以下操作使呼叫同步。

var result = User.findOne({"email": email}, _);
if (result === null) {
      console.log("ERROR" + err);
}

var id = result._id;
studentIds.push(id);
console.log("STUDENT: " + result);

Answer 2

解决方案是

var studentIds = [];
var loopRecords = function (records, cb) {
 if (!records.length) return cb();
 var email = records.shift();
 User.findOne({"email": email}, (err, student) = > {
    if (err) {
        console.log("ERROR" + err);
    }
    if (student) {
        var id = student._id;
        studentIds.push(id);
        console.log("STUDENT: " + student);
    }
    return loopRecords(records, cb);
 }
}

loopRecords(studentEmails,function () {
  console.log('END');
})

MongoDB查询for循环问题

2 个答案: