Почему возникает ошибка Cannot set property 'user' of undefined?

Question

Сергей Беловенцев @Sergalas

Учусь

Node.js

Почему возникает ошибка Cannot set property 'user' of undefined?

at /var/www/html/chatnode/routes/login.js:24:26
        at /var/www/html/chatnode/node_modules/async/dist/async.js:486:20
        at nextTask (/var/www/html/chatnode/node_modules/async/dist/async.js:5008:33)
        at /var/www/html/chatnode/node_modules/async/dist/async.js:5015:17
        at /var/www/html/chatnode/node_modules/async/dist/async.js:339:31
        at /var/www/html/chatnode/node_modules/async/dist/async.js:847:20
        at /var/www/html/chatnode/models/user.js:53:11
        at nextTask (/var/www/html/chatnode/node_modules/async/dist/async.js:5021:18)
        at /var/www/html/chatnode/node_modules/async/dist/async.js:5015:17
        at /var/www/html/chatnode/node_modules/async/dist/async.js:339:31
        at /var/www/html/chatnode/node_modules/async/dist/async.js:847:20
        at Query.<anonymous> (/var/www/html/chatnode/node_modules/mongoose/lib/model.js:3327:16)
        at /var/www/html/chatnode/node_modules/kareem/index.js:259:21
        at /var/www/html/chatnode/node_modules/kareem/index.js:127:16
        at nextTickCallbackWith0Args (node.js:419:9)
        at process._tickCallback (node.js:348:13)

вот routes/login.js

var User = require('models/user').User;
    var HttpError = require('error').HttpError;
    var AuthError = require('models/user').AuthError;
    var async = require('async');
    
    exports.get = function(req, res) {
      res.render('login');
    };
    
    exports.post = function(req, res, next) {
      var username = req.body.username;
      var password = req.body.password;
    
    
        User.authorize(username, password, function(err, user) {
            if (err) {
                if (err instanceof AuthError) {
                    return next(new HttpError(403, err.message));
                } else {
                    return next(err);
                }
            }
    
            req.session.user = User._id;
            res.send({});
    
        });
    
    };

ругается на эту строку

req.session.user = User._id;

и на этот участок кода в models/user

schema.statics.authorize = function (username, password, callback) {
      
    
        var User = this;
          async.waterfall([
            function (callback) {
              User.findOne({username:username},callback);
            },
            function (user,callback) {
              if(user){
                if(user.checkPassword(password)){
                  callback(null, user);
                }else{
                  callback(new AuthError("Пароль неверен"));
                }
              }else{
                var user = new User({username:username,password:password})
                user.save(function (err) {
                  if(err)return callback(err);
                  callback(null, user);
                })
              }
            }
          ],callback);
        };
        exports.User = mongoose.model('User', schema);

ругается на эту часть

if(user){
            if(user.checkPassword(password)){
              callback(null, user);
            }else{
              callback(new AuthError("Пароль неверен"));
            }

сразу оговорюсь я учусь по этому скринкасту