Node.js错误：发送标头后无法设置标头。

Question

我正在编写一个简单的查询以获取mongodb中所有课程列表。 我得到一个{}作为响应，node.js停止并显示错误，“错误：发送标头后无法设置标头”。

这是我的课程收集模式。 course.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var course = new Schema({

course : [{courseName : String}]

}, {collection : 'course'});

module.exports = mongoose.model('course', course);

这是用于查询的index.js文件中的router.get。

router.get('/course', function (req, res) {

  collection = db.collection('course');
  query = {};
   projection = {"_id" : 0, "course.courseName": 1};

  cursor = collection.find(query);
  cursor.project(projection);
  cursor.forEach(function (doc) {
          res.send(doc);
      },
      function (err) {
          if(err)
              throw err;
      }
  );

});

这是我得到的错误。

GET /course 200 34.808 ms - 2
/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:175
  throw err;
  ^

Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:356:11)
at ServerResponse.header (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/express/lib/response.js:718:10)
at ServerResponse.send (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/express/lib/response.js:163:12)
at ServerResponse.json (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/express/lib/response.js:249:15)
at ServerResponse.send (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/express/lib/response.js:151:21)
at /home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/routes/index.js:234:19
at /home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/lib/cursor.js:773:23
at handleCallback (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:172:5)
at nextFunction (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:682:5)
at Cursor.next [as _next] (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:692:3)
at loop (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/lib/cursor.js:687:8)
at _each (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/lib/cursor.js:734:16)
at /home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/lib/cursor.js:745:7
at handleCallback (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/lib/utils.js:96:12)
at /home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/lib/cursor.js:675:5
at handleCallback (/home/vikas/study/angularjs/learnAngularjsCoursera/passportAuthentication/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:172:5)

请向我建议我要去哪里错了。

Answer 1

使用express，您只能在每个路由上拨打一次res.send 。 就您而言，您似乎正在尝试将文档列表返回给客户端，对吗？

您需要做的是先创建一个对象数组，然后返回它。

您可以通过以下方法修改现有路线以使其正常运行：

router.get('/course', function(req, res) {
  var collection = db.collection('course');
  var query = {};
  var projection = {"_id" : 0, "course.courseName": 1};
  var docs = [];

  cursor = collection.find(query);
  cursor.project(projection);

  cursor.forEach(function(doc) {
    docs.push(doc);
  }, function (err) {
    if (err) throw err;
    res.json({docs: docs});
  });
});

Answer 2

可以尝试使用回调函数，并return res.send所有数据的return res.send 。

喜欢：

router.get('/course', function (req, res) {

  var collection = db.collection('course');
  var query = {};
  var projection = {"_id" : 0, "course.courseName": 1};

  collection.find(query, projection).toArray(function (err, docs) {
      if(err) {
           return res.status(500).send({error:err.message});
      } 
      return res.status(200).send(docs);
  });

});

您可以根据问题描述使用猫鼬。 所以您应该使用它而不是collection = db.collection('course'); 。 首先加载该course模型，然后使用它

var Course = require("course");// your course.js file path. can be like: "../model/course"

Course.find(query, projection, function (err, docs){
//... like above function 
});