Having an issue getting user id - node.js

Question

So I am working on building a web app and I have a lot of it working. However I am trying to link my user._id to my items database. But when I try to post an item using postman the app crashes saying it cant read property '_id' of null. I know I am missing something but I honestly can't figure out what other code I need to implement. Any help would be great. Thanks
Here is the code for the UserSchema:
const mongoose = require('mongoose');
const passportLocalMongoose = require("passport-local-mongoose");
const UserSchema = new mongoose.Schema({
username: {
type: String,
trim: true,
unique: true,
required: true,
minlength: 3,
maxlength: 15
},
firstName: {
type: String,
required: true,
minlength: 3,
maxlength: 15
},
lastName: {
type: String,
required: true,
minlength: 3,
maxlength: 15
},
email: {
type: String,
unique: true,
required: true
},
resetPasswordToken: String,
resetPasswordExpires: Date,
isAdmin: {
type: Boolean,
default: false
}
});
UserSchema.plugin(passportLocalMongoose);
module.exports = mongoose.model("user", UserSchema);
Here is the code for the ItemSchema:
const mongoose = require('mongoose');
const User = require('./user');
const ItemSchema = new mongoose.Schema({
name: {
type: String,
required: true,
minlength: 3,
maxlength: 20
},
description: {
type: String,
required: true
},
price: {
type: Number,
required: true
},
createdBy: {
id: {
type: mongoose.Schema.Types.ObjectId,
ref: "User"
},
username: String
},
createdAt: {
type: Date,
default: Date.now
}
});
module.exports = mongoose.model("items", ItemSchema);
And here is the code for the route thats throwing the error:
const express = require("express");
const router = express.Router();
const User = require("../models/user");
router.route("/item/add")
.post(function(req, res) {
User.findById(req.user._id, function(user, err) {
if (err) {
console.log(err);
}
var item = new Item();
item.name = req.body.name;
item.description = req.body.description;
item.price = req.body.price;
item.createdBy = { id: req.user._id, username: req.user.username };
item.save(function(err) {
if (err) {
res.send(err);
}
res.json({ message: "Item was successfully saved" });
console.log(item);
});
});
});

Answer 1

You need to send your data in json format in postman for example:
{'id':1, 'name':'jhon doe', 'email':'jhondoe#example.com'}
in your backend file you need to call
req.body.id not req.user._id