Saves only the first for item in the database

Question

I am using Postman to insert some data(structure below) in the database via POST, but it is only saving the first item of the list, someone can help me, please?

Structure of the POST:

{
    "products": [
        {
            "id": 1,
            "quantity": 17
        },
        {
            "id": 2,
            "quantity": 2
        }
    ]
}

py.models

from django.db import models

class Checkout(models.Model):
    pedido = models.IntegerField(primary_key=True, verbose_name="ID")
    total_amount = models.DecimalField(max_digits=50, decimal_places=2, default=0.00, 
    verbose_name='Valor total')
    total_amount_with_discount = models.DecimalField(max_digits=50, decimal_places=2, default=0.00,  verbose_name='Valor total com desconto')
    total_discount = models.DecimalField(max_digits=50, decimal_places=2, default=0.00,     verbose_name='Desconto total')

    class Meta:
        verbose_name = 'Pedidos'
        verbose_name_plural = 'Pedidos'

    def __str__(self):
        return 'Pedido: {}'.format(self.pedido)


class CartItem(models.Model):

    pedido_id = models.IntegerField()
    product_id = models.IntegerField()
    quantity = models.CharField(max_length=5, verbose_name='Quantidade', default=1, blank=True, null=True)
    line_item_total = models.CharField(max_length=5, verbose_name='Total', blank=True, null=True)

    class Meta:
        verbose_name = 'Itens dos Checkout'

    def __str__(self):
        return '{} - {}'.format(self.product_id, self.quantity)

py views.

from django.http import HttpResponse
from django.views.decorators.csrf import csrf_exempt
from rest_framework.renderers import JSONRenderer
from rest_framework.parsers import JSONParser

from apps.checkout.models import Checkout, CartItem
from apps.checkout.serializers import  CartItemSerializer


class JSONResponse(HttpResponse):
    def __init__(self, data, **kwargs):
        content = JSONRenderer().render(data)
        kwargs['content_type'] = 'application/json'
        super(JSONResponse, self).__init__(content, **kwargs)


@csrf_exempt
def checkout(request):
    if request.method == 'GET':
        itens = CartItem.objects.all()
        itensSerializer = CartItemSerializer(itens, many=True)
        return JSONResponse(itensSerializer.data)


    elif request.method == 'POST':
        data = JSONParser().parse(request)
        cartItem = CartItem()


        for item in data['products']:

            product_id = item['id']
            product_quantity = item['quantity']

            cartItem.pedido = checkout.pedido
            cartItem.product_id = product_id
            cartItem.quantity = product_quantity

            unit_amount = 100

            cartItem.line_item_total = unit_amount * product_quantity
            cartItem.save()

            itens = CartItem.objects.all()
            serializer = CartItemSerializer(itens, many=True)

            return JSONResponse(serializer.data)

py serializers.

from rest_framework import serializers
from .models import CartItem


class CartItemSerializer(serializers.ModelSerializer):
    class Meta:
        model = CartItem
        fields = ['id', 'product_id', 'quantity']