未捕获的TypeError：无法读取未定义的属性“ _id”

Question

我收到TypeError：尝试根据用户ID获取用户时，无法读取未定义的属性'_id'。

我的路线和控制器看起来像这样：

// routes.js

var AuthenticationController = require('./controllers/authentication');

module.exports = function (app){
    var apiRoutes = express.Router();
    apiRoutes.get('/user/:_id', AuthenticationController.getUserById);
    app.use('/api', apiRoutes);
};

//控制器

 exports.getUserById = function (req, res, next){
        User.findById(
            {_id: req.params._id}, function(err,users){
                if (err){
                    res.send(err);
                }
                res.status(200).json({
                    Success: true,
                    user: users
                });
            });
};

下面是我用于按ID获取用户的单元测试代码。

// user-test.spec.js

describe('/GET/:_id user', () => {
    it('it should get a user by the given ID', (done) => {
        var user = new User({ email: "mjn.nilesh12@gmail.com", password: "gunners", fullname: "Nilesh Maharjan"});
        user.save((err, user) => {
            chai.request(server)
            .get('api/user/' + user._id)
            .send(user)
            .end((err, res) => {
                res.should.have.status(200);
            done();
            });
        });
    });
});

//完全错误

Uncaught TypeError: Cannot read property '_id' of undefined
      at user.save (test/user-test.spec.js:73:38)
      at /Users/seva/Desktop/project1/node_modules/mongoose/lib/model.js:4670:16
      at /Users/seva/Desktop/project1/node_modules/mongoose/lib/utils.js:250:11
      at $__save.error (node_modules/mongoose/lib/model.js:476:16)
      at /Users/seva/Desktop/project1/node_modules/kareem/index.js:246:48
      at next (node_modules/kareem/index.js:167:27)
      at next (node_modules/kareem/index.js:169:9)
      at Kareem.execPost (node_modules/kareem/index.js:217:3)
      at _handleWrapError (node_modules/kareem/index.js:245:21)
      at _cb (node_modules/kareem/index.js:304:16)
      at /Users/seva/Desktop/project1/node_modules/mongoose/lib/model.js:329:9
      at /Users/seva/Desktop/project1/node_modules/kareem/index.js:135:16
      at _combinedTickCallback (internal/process/next_tick.js:132:7)
      at process._tickCallback (internal/process/next_tick.js:181:9)