From d9cb44e93c827006e52a50b326af7cdeffc62472 Mon Sep 17 00:00:00 2001
From: Brian Rogers <captbrogers@protonmail.ch>
Date: Fri, 10 Apr 2026 08:54:20 -0600
Subject: [PATCH] adding a cart

---
 app/Http/Controllers/CartController.php       |  78 +++++++++
 app/Http/Requests/StoreCartRequest.php        |  24 +++
 app/Http/Requests/UpdateCartRequest.php       |  24 +++
 app/Models/Cart.php                           |  27 ++++
 app/Models/User.php                           |   6 +
 bootstrap/app.php                             |   5 +
 database/factories/CartFactory.php            |  24 +++
 .../2026_04_10_135926_create_carts_table.php  |  29 ++++
 ...04_10_140658_create_cart_product_table.php |  29 ++++
 ...2833_add_unique_user_id_to_carts_table.php |  28 ++++
 ...8_make_user_id_nullable_on_carts_table.php |  30 ++++
 resources/js/pages/Home.vue                   | 151 ++++++++++++++----
 routes/api.php                                |  10 +-
 routes/web.php                                |  18 ++-
 .../Http/Controllers/CartControllerTest.php   |  77 +++++++++
 tests/Feature/Models/CartTest.php             |  38 +++++
 16 files changed, 559 insertions(+), 39 deletions(-)
 create mode 100644 app/Http/Controllers/CartController.php
 create mode 100644 app/Http/Requests/StoreCartRequest.php
 create mode 100644 app/Http/Requests/UpdateCartRequest.php
 create mode 100644 app/Models/Cart.php
 create mode 100644 database/factories/CartFactory.php
 create mode 100644 database/migrations/2026_04_10_135926_create_carts_table.php
 create mode 100644 database/migrations/2026_04_10_140658_create_cart_product_table.php
 create mode 100644 database/migrations/2026_04_10_142833_add_unique_user_id_to_carts_table.php
 create mode 100644 database/migrations/2026_04_10_144128_make_user_id_nullable_on_carts_table.php
 create mode 100644 tests/Feature/Http/Controllers/CartControllerTest.php
 create mode 100644 tests/Feature/Models/CartTest.php

diff --git a/app/Http/Controllers/CartController.php b/app/Http/Controllers/CartController.php
new file mode 100644
index 0000000..9744cce
--- /dev/null
+++ b/app/Http/Controllers/CartController.php
@@ -0,0 +1,78 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Http\Requests\StoreCartRequest;
+use App\Http\Requests\UpdateCartRequest;
+use App\Models\Cart;
+use App\Models\Product;
+use Illuminate\Http\JsonResponse;
+use Illuminate\Http\Request;
+
+class CartController extends Controller
+{
+    public function store(StoreCartRequest $request): JsonResponse
+    {
+        $cart = $this->resolveCart();
+
+        $existing = $cart->products()->wherePivot('product_id', $request->product_id)->first();
+
+        if ($existing) {
+            $cart->products()->updateExistingPivot($request->product_id, [
+                'quantity' => $existing->pivot->quantity + 1,
+            ]);
+        } else {
+            $cart->products()->attach($request->product_id, ['quantity' => 1]);
+        }
+
+        return response()->json($this->cartItems($cart));
+    }
+
+    public function update(UpdateCartRequest $request, Product $product): JsonResponse
+    {
+        $cart = $this->resolveCart();
+
+        $cart->products()->updateExistingPivot($product->id, [
+            'quantity' => $request->quantity,
+        ]);
+
+        return response()->json($this->cartItems($cart));
+    }
+
+    public function destroy(Request $request, Product $product): JsonResponse
+    {
+        $cart = $this->resolveCart();
+
+        $cart->products()->detach($product->id);
+
+        return response()->json($this->cartItems($cart));
+    }
+
+    private function resolveCart(): Cart
+    {
+        $cartId = session('cart_id');
+
+        if ($cartId) {
+            $cart = Cart::find($cartId);
+            if ($cart) {
+                return $cart;
+            }
+        }
+
+        $cart = Cart::create(['user_id' => request()->user()?->id]);
+        session(['cart_id' => $cart->id]);
+
+        return $cart;
+    }
+
+    /**
+     * @return array<int, array{product_id: int, quantity: int}>
+     */
+    private function cartItems(Cart $cart): array
+    {
+        return $cart->products()->get()->map(fn ($p) => [
+            'product_id' => $p->id,
+            'quantity' => $p->pivot->quantity,
+        ])->all();
+    }
+}
diff --git a/app/Http/Requests/StoreCartRequest.php b/app/Http/Requests/StoreCartRequest.php
new file mode 100644
index 0000000..9a35f34
--- /dev/null
+++ b/app/Http/Requests/StoreCartRequest.php
@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Http\Requests;
+
+use Illuminate\Contracts\Validation\ValidationRule;
+use Illuminate\Foundation\Http\FormRequest;
+
+class StoreCartRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * @return array<string, ValidationRule|array<mixed>|string>
+     */
+    public function rules(): array
+    {
+        return [
+            'product_id' => ['required', 'integer', 'exists:products,id'],
+        ];
+    }
+}
diff --git a/app/Http/Requests/UpdateCartRequest.php b/app/Http/Requests/UpdateCartRequest.php
new file mode 100644
index 0000000..6e0ed5b
--- /dev/null
+++ b/app/Http/Requests/UpdateCartRequest.php
@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Http\Requests;
+
+use Illuminate\Contracts\Validation\ValidationRule;
+use Illuminate\Foundation\Http\FormRequest;
+
+class UpdateCartRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * @return array<string, ValidationRule|array<mixed>|string>
+     */
+    public function rules(): array
+    {
+        return [
+            'quantity' => ['required', 'integer', 'min:1'],
+        ];
+    }
+}
diff --git a/app/Models/Cart.php b/app/Models/Cart.php
new file mode 100644
index 0000000..171a002
--- /dev/null
+++ b/app/Models/Cart.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace App\Models;
+
+use Database\Factories\CartFactory;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
+
+class Cart extends Model
+{
+    /** @use HasFactory<CartFactory> */
+    use HasFactory;
+
+    protected $fillable = ['user_id'];
+
+    public function user(): BelongsTo
+    {
+        return $this->belongsTo(User::class);
+    }
+
+    public function products(): BelongsToMany
+    {
+        return $this->belongsToMany(Product::class)->withPivot('quantity');
+    }
+}
diff --git a/app/Models/User.php b/app/Models/User.php
index f40a71d..804e01c 100644
--- a/app/Models/User.php
+++ b/app/Models/User.php
@@ -7,6 +7,7 @@ use Database\Factories\UserFactory;
 use Illuminate\Database\Eloquent\Attributes\Fillable;
 use Illuminate\Database\Eloquent\Attributes\Hidden;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Relations\HasOne;
 use Illuminate\Foundation\Auth\User as Authenticatable;
 use Illuminate\Notifications\Notifiable;
 use Laravel\Fortify\TwoFactorAuthenticatable;
@@ -18,6 +19,11 @@ class User extends Authenticatable
     /** @use HasFactory<UserFactory> */
     use HasFactory, Notifiable, TwoFactorAuthenticatable;
 
+    public function cart(): HasOne
+    {
+        return $this->hasOne(Cart::class);
+    }
+
     /**
      * Get the attributes that should be cast.
      *
diff --git a/bootstrap/app.php b/bootstrap/app.php
index 34a6bb8..4905b14 100644
--- a/bootstrap/app.php
+++ b/bootstrap/app.php
@@ -6,6 +6,7 @@ use Illuminate\Foundation\Application;
 use Illuminate\Foundation\Configuration\Exceptions;
 use Illuminate\Foundation\Configuration\Middleware;
 use Illuminate\Http\Middleware\AddLinkHeadersForPreloadedAssets;
+use Illuminate\Session\Middleware\StartSession;
 
 return Application::configure(basePath: dirname(__DIR__))
     ->withRouting(
@@ -17,6 +18,10 @@ return Application::configure(basePath: dirname(__DIR__))
     ->withMiddleware(function (Middleware $middleware): void {
         $middleware->encryptCookies(except: ['appearance', 'sidebar_state']);
 
+        $middleware->api(append: [
+            StartSession::class,
+        ]);
+
         $middleware->web(append: [
             HandleAppearance::class,
             HandleInertiaRequests::class,
diff --git a/database/factories/CartFactory.php b/database/factories/CartFactory.php
new file mode 100644
index 0000000..1c9579b
--- /dev/null
+++ b/database/factories/CartFactory.php
@@ -0,0 +1,24 @@
+<?php
+
+namespace Database\Factories;
+
+use App\Models\Cart;
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+/**
+ * @extends Factory<Cart>
+ */
+class CartFactory extends Factory
+{
+    /**
+     * Define the model's default state.
+     *
+     * @return array<string, mixed>
+     */
+    public function definition(): array
+    {
+        return [
+            'user_id' => \App\Models\User::factory(),
+        ];
+    }
+}
diff --git a/database/migrations/2026_04_10_135926_create_carts_table.php b/database/migrations/2026_04_10_135926_create_carts_table.php
new file mode 100644
index 0000000..0df4b94
--- /dev/null
+++ b/database/migrations/2026_04_10_135926_create_carts_table.php
@@ -0,0 +1,29 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('carts', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('user_id')->constrained();
+            $table->timestamp('stale_date')->nullable();
+            $table->timestamps();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('carts');
+    }
+};
diff --git a/database/migrations/2026_04_10_140658_create_cart_product_table.php b/database/migrations/2026_04_10_140658_create_cart_product_table.php
new file mode 100644
index 0000000..50f1f93
--- /dev/null
+++ b/database/migrations/2026_04_10_140658_create_cart_product_table.php
@@ -0,0 +1,29 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('cart_product', function (Blueprint $table) {
+            $table->foreignId('cart_id')->constrained()->cascadeOnDelete();
+            $table->foreignId('product_id')->constrained()->cascadeOnDelete();
+            $table->unsignedInteger('quantity')->default(1);
+            $table->primary(['cart_id', 'product_id']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('cart_product');
+    }
+};
diff --git a/database/migrations/2026_04_10_142833_add_unique_user_id_to_carts_table.php b/database/migrations/2026_04_10_142833_add_unique_user_id_to_carts_table.php
new file mode 100644
index 0000000..dd0235d
--- /dev/null
+++ b/database/migrations/2026_04_10_142833_add_unique_user_id_to_carts_table.php
@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('carts', function (Blueprint $table) {
+            $table->unique('user_id');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('carts', function (Blueprint $table) {
+            $table->dropUnique(['user_id']);
+        });
+    }
+};
diff --git a/database/migrations/2026_04_10_144128_make_user_id_nullable_on_carts_table.php b/database/migrations/2026_04_10_144128_make_user_id_nullable_on_carts_table.php
new file mode 100644
index 0000000..310454a
--- /dev/null
+++ b/database/migrations/2026_04_10_144128_make_user_id_nullable_on_carts_table.php
@@ -0,0 +1,30 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('carts', function (Blueprint $table) {
+            $table->dropUnique(['user_id']);
+            $table->foreignId('user_id')->nullable()->change();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('carts', function (Blueprint $table) {
+            $table->foreignId('user_id')->nullable(false)->change();
+            $table->unique('user_id');
+        });
+    }
+};
diff --git a/resources/js/pages/Home.vue b/resources/js/pages/Home.vue
index 481fd01..1fb9ab7 100644
--- a/resources/js/pages/Home.vue
+++ b/resources/js/pages/Home.vue
@@ -1,5 +1,8 @@
 <script setup lang="ts">
+import { computed, ref } from 'vue';
 import { Head } from '@inertiajs/vue3';
+import { useHttp } from '@inertiajs/vue3';
+import { store, update, destroy } from '@/actions/App/Http/Controllers/CartController';
 
 interface Product {
     id: number;
@@ -9,12 +12,56 @@ interface Product {
     image_url: string | null;
 }
 
-defineProps<{
+interface CartItem {
+    product_id: number;
+    quantity: number;
+}
+
+const props = defineProps<{
     products: Product[];
+    cart: CartItem[];
 }>();
 
+const cartItems = ref<CartItem[]>(props.cart);
+
+const addHttp = useHttp({ product_id: 0 });
+const removeHttp = useHttp({});
+
+function addToCart(product: Product): void {
+    addHttp.product_id = product.id;
+    addHttp.post(store.url(), {
+        onSuccess: (data: unknown) => {
+            cartItems.value = data as CartItem[];
+        },
+    });
+}
+
+function removeFromCart(productId: number): void {
+    removeHttp.delete(destroy.url(productId), {
+        onSuccess: (data: unknown) => {
+            cartItems.value = data as CartItem[];
+        },
+    });
+}
+
+const cartWithDetails = computed(() =>
+    cartItems.value.map((item) => ({
+        ...item,
+        product: props.products.find((p) => p.id === item.product_id)!,
+    })),
+);
+
+const subtotal = computed(() =>
+    cartWithDetails.value.reduce(
+        (sum, item) => sum + item.product.price_cents * item.quantity,
+        0,
+    ),
+);
+
 function formatPrice(cents: number): string {
-    return new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' }).format(cents / 100);
+    return new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' }).format(
+        cents / 100,
+    );
 }
 </script>
 
@@ -23,39 +70,79 @@ function formatPrice(cents: number): string {
         <link rel="preconnect" href="https://rsms.me/" />
         <link rel="stylesheet" href="https://rsms.me/inter/inter.css" />
     </Head>
-    <div class="min-h-screen bg-[#FDFDFC] p-6 text-[#1b1b18] dark:bg-[#0a0a0a] dark:text-[#FDFDFC] lg:p-8">
-        <header class="mx-auto mb-8 max-w-4xl">
-            <h1 class="text-2xl font-semibold">Packages</h1>
-        </header>
-        <main class="mx-auto max-w-4xl">
-            <p v-if="products.length === 0" class="text-gray-500">No packages available.</p>
-            <div v-else class="grid grid-cols-1 gap-6 sm:grid-cols-2 lg:grid-cols-3">
-                <div
-                    v-for="product in products"
-                    :key="product.id"
-                    class="flex flex-col overflow-hidden rounded-lg border border-gray-200 bg-white shadow-sm dark:border-gray-700 dark:bg-gray-800"
-                >
-                    <img
-                        v-if="product.image_url"
-                        :src="product.image_url"
-                        :alt="product.name"
-                        class="h-48 w-full object-cover"
-                    />
-                    <div v-else class="flex h-48 items-center justify-center bg-gray-100 dark:bg-gray-700">
-                        <span class="text-sm text-gray-400">No image</span>
-                    </div>
-                    <div class="flex flex-1 flex-col p-5">
-                        <h2 class="mb-1 font-medium capitalize">{{ product.name }}</h2>
-                        <p class="mb-4 grow text-sm text-gray-500 dark:text-gray-400">{{ product.description }}</p>
-                        <div class="flex items-center justify-between">
-                            <p class="text-lg font-semibold">{{ formatPrice(product.price_cents) }}</p>
-                            <button type="button" class="rounded-md bg-gray-900 px-3 py-1.5 text-sm font-medium text-white hover:bg-gray-700 dark:bg-white dark:text-gray-900 dark:hover:bg-gray-200">
-                                Add
-                            </button>
+    <div class="flex min-h-screen bg-[#FDFDFC] text-[#1b1b18] dark:bg-[#0a0a0a] dark:text-[#FDFDFC]">
+        <div class="flex-1 p-6 lg:p-8">
+            <header class="mb-8">
+                <h1 class="text-2xl font-semibold">Packages</h1>
+            </header>
+            <main>
+                <p v-if="products.length === 0" class="text-gray-500">No packages available.</p>
+                <div v-else class="grid grid-cols-1 gap-6 sm:grid-cols-2 lg:grid-cols-3">
+                    <div
+                        v-for="product in products"
+                        :key="product.id"
+                        class="flex flex-col overflow-hidden rounded-lg border border-gray-200 bg-white shadow-sm dark:border-gray-700 dark:bg-gray-800"
+                    >
+                        <img
+                            v-if="product.image_url"
+                            :src="product.image_url"
+                            :alt="product.name"
+                            class="h-96 w-full object-cover"
+                        />
+                        <div v-else class="flex h-48 items-center justify-center bg-gray-100 dark:bg-gray-700">
+                            <span class="text-sm text-gray-400">No image</span>
+                        </div>
+                        <div class="flex flex-1 flex-col p-5">
+                            <h2 class="mb-1 font-medium capitalize">{{ product.name }}</h2>
+                            <p class="mb-4 grow text-sm text-gray-500 dark:text-gray-400">{{ product.description }}</p>
+                            <div class="flex items-center justify-between">
+                                <p class="text-lg font-semibold">{{ formatPrice(product.price_cents) }}</p>
+                                <button
+                                    type="button"
+                                    :disabled="addHttp.processing"
+                                    class="rounded-md bg-gray-900 px-3 py-1.5 text-sm font-medium text-white hover:bg-gray-700 disabled:opacity-50 dark:bg-white dark:text-gray-900 dark:hover:bg-gray-200"
+                                    @click="addToCart(product)"
+                                >
+                                    Add
+                                </button>
+                            </div>
                         </div>
                     </div>
                 </div>
+            </main>
+        </div>
+
+        <aside class="w-[300px] shrink-0 border-l border-gray-200 p-6 dark:border-gray-700">
+            <h2 class="mb-4 text-lg font-semibold">Cart</h2>
+            <p v-if="cartItems.length === 0" class="text-sm text-gray-500">Your cart is empty.</p>
+            <ul v-else class="mb-6 space-y-3">
+                <li
+                    v-for="item in cartWithDetails"
+                    :key="item.product_id"
+                    class="flex items-center justify-between text-sm"
+                >
+                    <span class="capitalize">
+                        {{ item.product.name }}
+                        <span class="text-gray-400">×{{ item.quantity }}</span>
+                    </span>
+                    <div class="flex items-center gap-2">
+                        <span class="font-medium">{{ formatPrice(item.product.price_cents * item.quantity) }}</span>
+                        <button
+                            type="button"
+                            class="text-gray-400 hover:text-red-500"
+                            @click="removeFromCart(item.product_id)"
+                        >
+                            &times;
+                        </button>
+                    </div>
+                </li>
+            </ul>
+            <div class="border-t border-gray-200 pt-4 dark:border-gray-700">
+                <div class="flex items-center justify-between font-semibold">
+                    <span>Subtotal</span>
+                    <span>{{ formatPrice(subtotal) }}</span>
+                </div>
             </div>
-        </main>
+        </aside>
     </div>
 </template>
diff --git a/routes/api.php b/routes/api.php
index ca0bca0..252122f 100644
--- a/routes/api.php
+++ b/routes/api.php
@@ -1,9 +1,11 @@
 <?php
 
-use Illuminate\Http\Request;
+use App\Http\Controllers\CartController;
 use Illuminate\Support\Facades\Route;
 
-Route::get('/cart', function (Request $request) {
-    return $request->user()->cart();
+//middleware('auth:sanctum')->
+Route::group([], function () {
+    Route::post('/cart', [CartController::class, 'store'])->name('cart.store');
+    Route::patch('/cart/{product}', [CartController::class, 'update'])->name('cart.update');
+    Route::delete('/cart/{product}', [CartController::class, 'destroy'])->name('cart.destroy');
 });
-//})->middleware('auth:sanctum');
diff --git a/routes/web.php b/routes/web.php
index 088b46c..f894735 100644
--- a/routes/web.php
+++ b/routes/web.php
@@ -1,5 +1,6 @@
 <?php
 
+use App\Models\Cart;
 use App\Models\Product;
 use Illuminate\Support\Facades\Route;
 use Laravel\Fortify\Features;
@@ -8,9 +9,20 @@ use Laravel\Fortify\Features;
     'canRegister' => Features::enabled(Features::registration()),
 ])->name('home');*/
 
-Route::get('/', fn () => inertia('Home', [
-    'products' => Product::all(),
-]))->name('home');
+Route::get('/', function () {
+    $cartId = session('cart_id');
+    $cart = $cartId ? Cart::with('products')->find($cartId) : null;
+
+    return inertia('Home', [
+        'products' => Product::all(),
+        'cart' => $cart
+            ? $cart->products->map(fn ($p) => [
+                'product_id' => $p->id,
+                'quantity' => $p->pivot->quantity,
+            ])
+            : [],
+    ]);
+})->name('home');
 
 Route::middleware(['auth', 'verified'])->group(function () {
     Route::inertia('dashboard', 'Dashboard')->name('dashboard');
diff --git a/tests/Feature/Http/Controllers/CartControllerTest.php b/tests/Feature/Http/Controllers/CartControllerTest.php
new file mode 100644
index 0000000..0fc2231
--- /dev/null
+++ b/tests/Feature/Http/Controllers/CartControllerTest.php
@@ -0,0 +1,77 @@
+<?php
+
+use App\Models\Cart;
+use App\Models\Product;
+use App\Models\User;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+
+uses(RefreshDatabase::class);
+
+test('guests can add to cart without being authenticated', function () {
+    $product = Product::factory()->create();
+
+    $this->postJson('/api/cart', ['product_id' => $product->id])
+        ->assertSuccessful()
+        ->assertJsonFragment(['product_id' => $product->id, 'quantity' => 1]);
+});
+
+test('adding a product creates a cart and returns cart items', function () {
+    $product = Product::factory()->create();
+
+    $response = $this->postJson('/api/cart', ['product_id' => $product->id]);
+
+    $response->assertSuccessful()
+        ->assertJsonFragment(['product_id' => $product->id, 'quantity' => 1]);
+
+    expect(Cart::count())->toBe(1);
+});
+
+test('adding the same product increments quantity', function () {
+    $product = Product::factory()->create();
+
+    $this->postJson('/api/cart', ['product_id' => $product->id]);
+    $response = $this->postJson('/api/cart', ['product_id' => $product->id]);
+
+    $response->assertSuccessful()
+        ->assertJsonFragment(['product_id' => $product->id, 'quantity' => 2]);
+});
+
+test('adding a non-existent product fails validation', function () {
+    $this->postJson('/api/cart', ['product_id' => 9999])
+        ->assertUnprocessable();
+});
+
+test('updating a cart item changes its quantity', function () {
+    $product = Product::factory()->create();
+    $cart = Cart::factory()->create(['user_id' => null]);
+    $cart->products()->attach($product->id, ['quantity' => 1]);
+
+    $response = $this->withSession(['cart_id' => $cart->id])
+        ->patchJson("/api/cart/{$product->id}", ['quantity' => 5]);
+
+    $response->assertSuccessful()
+        ->assertJsonFragment(['product_id' => $product->id, 'quantity' => 5]);
+});
+
+test('removing a product detaches it from the cart', function () {
+    $product = Product::factory()->create();
+    $cart = Cart::factory()->create(['user_id' => null]);
+    $cart->products()->attach($product->id, ['quantity' => 2]);
+
+    $response = $this->withSession(['cart_id' => $cart->id])
+        ->deleteJson("/api/cart/{$product->id}");
+
+    $response->assertSuccessful()
+        ->assertJsonMissing(['product_id' => $product->id]);
+
+    expect($cart->fresh()->products)->toHaveCount(0);
+});
+
+test('a cart can optionally belong to a user', function () {
+    $user = User::factory()->create();
+    $cartWithUser = Cart::factory()->create(['user_id' => $user->id]);
+    $cartGuest = Cart::factory()->create(['user_id' => null]);
+
+    expect($cartWithUser->user)->toBeInstanceOf(User::class);
+    expect($cartGuest->user)->toBeNull();
+});
diff --git a/tests/Feature/Models/CartTest.php b/tests/Feature/Models/CartTest.php
new file mode 100644
index 0000000..404fbea
--- /dev/null
+++ b/tests/Feature/Models/CartTest.php
@@ -0,0 +1,38 @@
+<?php
+
+use App\Models\Cart;
+use App\Models\Product;
+use App\Models\User;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+
+uses(RefreshDatabase::class);
+
+test('a cart belongs to a user when one is set', function () {
+    $user = User::factory()->create();
+    $cart = Cart::factory()->create(['user_id' => $user->id]);
+
+    expect($cart->user->id)->toBe($user->id);
+});
+
+test('a cart can exist without a user', function () {
+    $cart = Cart::factory()->create(['user_id' => null]);
+
+    expect($cart->user)->toBeNull();
+});
+
+test('a user has one cart', function () {
+    $user = User::factory()->create();
+    Cart::factory()->create(['user_id' => $user->id]);
+
+    expect($user->cart)->toBeInstanceOf(Cart::class);
+});
+
+test('a cart can have products with quantities', function () {
+    $cart = Cart::factory()->create(['user_id' => null]);
+    $product = Product::factory()->create();
+
+    $cart->products()->attach($product->id, ['quantity' => 3]);
+
+    expect($cart->products)->toHaveCount(1);
+    expect($cart->products->first()->pivot->quantity)->toBe(3);
+});