Front end:

                 <div class="cart-table table-responsive">
                    <table class="table table-bordered">
                        <thead>
                        <tr>
                            <th class="pro-thumbnail"><input type="checkbox" id="select-all" style="margin-right: 20px;"></th>
                            <th class="pro-title"></th>
                            <th class="pro-price"></th>
                            <th class="pro-quantity"></th>
                            <th class="pro-subtotal"></th>
                            <th class="pro-remove"></th>
                        </tr>
                        </thead>
                        <tbody>
                        @foreach($cartItems as $item)
                        <tr>
                            <td class="pro-thumbnail">
                                <input type="checkbox" name="select" value="{{ $item->productSku->id }}" {{ $item->productSku->on_slae ? "checked" : "disabled" }}>
                                <a href="-sharp">
                                    <img class="img-fluid" src="{{ $item->productSku->product->image_url }}" alt="Product">
                                </a>
                            </td>
                            <td class="pro-title">
                                <a href="-sharp">
                                    {{ $item->productSku->product->title }}
                                </a>
                                <div @if(!$item->productSku->product->on_sale) class="not_on_sale" @endif>
                                    <span class="product_title">
                                        <a target="_blank" href="{{ route("products.show", [$item->productSku->product_id]) }}" style="text-align: center">
                                            {{ $item->productSku->title }}
                                        </a>
                                    </span>
                                    @if(!$item->productSku->product->on_sale)
                                        <span class="warning"></span>
                                    @endif
                                </div>
                            </td>
                            <td class="pro-price"><span>{{ $item->productSku->price }}</span></td>
                            <td class="pro-quantity">
                                <div class="pro-qty">
                                    <input type="text"  @if(!$item->productSku->product->on_sale) disabled @endif name="amount" value="{{ $item->amount }}">

                                </div>
                            </td>
                            <td class="pro-subtotal"><span>{{ $item->productSku->price }}</span></td>
                            <td class="pro-remove"><a href="-sharp"><i class="fa fa-trash-o"></i></a></td>
                        </tr>
                       @endforeach
                        </tbody>
                    </table>
                </div>

Controller:

class CartController extends Controller
{

//
public function index(Request $request)
{
    $cartItems = $request->user()->cartItems()->with(["productSku.product"])->get();
    return view("carts.index",compact("cartItems"));
}
//
public function add(AddCartRequest $request)
{
    $user   = $request->user();
    $skuId  = $request->input("sku_id");
    $amount = $request->input("amount");

    // 
    if ($cart = $user->cartItems()->where("product_sku_id", $skuId)->first()) {

        // 
        $cart->update([
            "amount" => $cart->amount + $amount,
        ]);
    } else {

        // 
        $cart = new CartItem(["amount" => $amount]);
        $cart->user()->associate($user);
        $cart->productSku()->associate($skuId);
        $cart->save();
    }

    return [];
}

}

current database:
public function up ()

{
    Schema::create("cart_items", function (Blueprint $table) {
        $table->increments("id");
        $table->unsignedInteger("user_id")->comment("ID");
        // user_id users,, ,.
        $table->foreign("user_id")->references("id")->on("users")->onDelete("cascade");
        $table->unsignedInteger("product_sku_id")->comment(" SKU ID");
        $table->foreign("product_sku_id")->references("id")->on("product_skus")->onDelete("cascade");
        $table->unsignedInteger("amount")->comment("");
        //$table->timestamps();
});

effect:

is it necessary to add fields to the database-- "then write the corresponding method to the controller; or if there is any better way, please post it for some, thank you