How to save req.body object with a user ID - node.js

Question

I am trying to save the request.body object containing an authenticated user ID into a new collection called useritems.
below is the req.body object with the user ID
{contact: "90000023", item: "Bread", price: "50", id: "5f4acf21287c6226ec0855af"}
next i find user with the id "5f4acf21287c6226ec0855af"
User.findOne({_id: _id}, function(err, items){
console.log(req.body)
if (err) {
console.log('err', err);
res.status(500).send(err);
} else {
const newItem = new Item ({
name:items.name,
email:items.email,
contact:req.body.contact,
item:req.body.item,
price:req.body.price,
});
newItem.save(function (err, item) {
if (err) {
console.log(err);
} else {
res.send(item);
}
});
)
}
})
})
here is the output:
{
_id: 5f4e32006ce4d91a1cd811e2,// mongodb assigns new id. However, i still want the userID (5f4acf21287c6226ec0855af) persisted
name: 'Bernad James',
email: 'Ben#gmail.com',
contact: 90000023,
item: 'Bread',
price: 50
}
how do I make it such that I am able to maintain the userID after save in the item collection
//here is my Item schema
const ItemSchema = new Schema({
name: {
type: String,
required: true
},
email: {
type: String,
required: true
},
contact: {
type: Number,
required: true
},
item: {
type: String,
required: true
},
price: {
type: Number,
required: true
},
And my user schema
const UserSchema = new Schema({
name: {
type: String,
required: true
},
email: {
type: String,
required: true
},
password: {
type: String,
required: true
},
I want the userID persisted throughout so that i can always reference it to deal with particular authenticated logged in user