Ember-data hasMany performance/design

User can have multiple carts and every cart can have multiple products

{
    carts: [
        {
            id: 1,
            title: "my fav cart",
            couponCode: 454,
            products: [7,8,9]
        },
        {
            id: 2,
            title: "my second fav cart",
            couponCode: 966,
            products: [4,5,6]
        }
    ],
    cartItems: [
        {
            id: "7",
            quantiy: 2,
            isWeight: false
        }
    ]
}

CartModel = DS.Model.extend({
    title: DS.attr("string"),
    couponCode: DS.attr("string"),
    products: DS.hasMany("cartItem")
});

CartItemModel = DS.Model.extend({
    quantity: DS.attr("number"),
    isWeight: DS.attr("boolean")
});

let's say I want to display single cart and do this.store.find("cart", 1)

When I update and save a product ember sends PUT request to /cartItems/{productId} and I can't know which cart product belongs to unless I have cartId in every CartItem

I could make products embedded, but then I would have to send the whole cart of ~50-100 items, just to change quantity for one item

How should I implement this in order to have minimal JSON without repetitive fields and maintain the ability to send only one item through network when I want to update quantity or delete item