How to implement a shopping cart in rest API

I'm creating a rest API for my android app and I'm stuck at the cart portion of the API and I want this kinda output data:

{
"userEmail": adbd@xyz.com,
"items": [
    {
        "item_id": "d3e9d433-bf54-4567-b070-cb033cc27c63",
        "quantity": 1
    },
    {
        "item_id": "1a3d7141-1676-422d-bcd1-dbe7eb3a561c",
        "quantity": 1
    },
    {
        "item_id": "0f768f63-3953-4f7d-b30b-c61fc2b07cc1",
        "quantity": 2
    }
]
}

But I don't know how to implement this I have used MongoDB and node js. Schema of my cart looks like this

var mongoose = require("mongoose");

var Cart = mongoose.Schema({

userEmailId: {
    type: String,
    default: "",
    required: true
},
items :[{
    itemIds: {
        type: String,
        default: "",
        required: true
    },
    itemQuantity: {
        type: String,
        default: "1",
        required: false
    }
}]
});

module.exports = mongoose.model("Cart", Cart);

Here is the controller of the cart:

const cart = require('../models/Cart_Model');

exports.showCart = (req,res) =>{

cart.find({
    userEmailId : req.params.userEmailId
},
function(err,Cart){

    if(err){
        res.send(err);
    }else{
        res.send(Cart);
    }
});
}

exports.updateItem = (req,res)=>{

var fieldsForUpdate = {};

res.send(req.body.itemIds);

cart.updateOne({
    userEmailId : req.body.userEmailId
},fieldsForUpdate,
function(err,num,raw){

});

}

But now I don't know how to proceed to add and update data to this cart and is this schema is okay? Thank you in advance! :)