Node.js 主体参数有时会变得未定义

我的代码如下所示：

user.js

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

var user = new Schema({
    login_type:Number,   // 0 :facebook, 1: google
    profile_img:String,
    ph_no:Number,
    user_name:String,
    email_id:{type:String,required:true},
},{ versionKey: false });


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

logtest.js

var userData = function(req, res) {
    upload = multer({
        limits: {
            fileSize: 1000000,
            files: 1
        },
        storage: storage,
        fileFilter: function(req, file, callback) {
            var ext = path.extname(file.originalname)
            if (ext !== '.png' && ext !== '.jpg' && ext !== '.gif' && ext !== '.jpeg') {
                return callback(res.end('Only images are allowed'), null)
            }
            callback(null, true);
        }
    }).single('profile_img');
    upload(req, res, function(err) {
        const user_name = req.body.user_name;
        console.log("user name is "+ user_name);

.......

......
}

所以，这里发生的是，除了user\u name之外的所有参数都来自请求，但不知何故，user\u name有时是未定义的。那么，是否需要进行任何修改