Use cartId from REST URL

app/code/Magento/Quote/Model/Webapi/SaveCartItemWithCartId.php

@@ -0,0 +1,41 @@
+<?php
+/**
+ *
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Quote\Model\Webapi;
+
+use Magento\Quote\Api\CartItemRepositoryInterface;
+use Magento\Quote\Api\Data\CartItemInterface;
+
+/**
+ * CartItemRepository extension to save a cart item for a specific guest cart
+ */
+class SaveCartItemWithCartId implements SaveCartItemWithCartIdInterface
+{
+    /**
+     * @var CartItemRepositoryInterface
+     */
+    private $cartItemRepository;
+
+    /**
+     * Constructor.
+     *
+     * @param CartItemRepositoryInterface $cartItemRepository
+     */
+    public function __construct(CartItemRepositoryInterface $cartItemRepository)
+    {
+        $this->cartItemRepository = $cartItemRepository;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function saveForCart($cartId, CartItemInterface $cartItem)
+    {
+        $cartItem->setQuoteId($cartId);
+
+        return $this->cartItemRepository->save($cartItem);
+    }
+}

app/code/Magento/Quote/Model/Webapi/SaveCartItemWithCartIdInterface.php

@@ -0,0 +1,24 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Quote\Model\Webapi;
+
+/**
+ * Interface SaveCartItemWithCartIdInterface
+ */
+interface SaveCartItemWithCartIdInterface
+{
+    /**
+     * Add/update the specified cart item for given cartId.
+     *
+     * @param int $cartId The cart ID.
+     * @param \Magento\Quote\Api\Data\CartItemInterface $cartItem The item.
+     * @return \Magento\Quote\Api\Data\CartItemInterface Item.
+     * @throws \Magento\Framework\Exception\NoSuchEntityException The specified cart does not exist.
+     * @throws \Magento\Framework\Exception\CouldNotSaveException The specified item could not be saved to the cart.
+     * @throws \Magento\Framework\Exception\InputException The specified item or cart is not valid.
+     */
+    public function saveForCart($cartId, \Magento\Quote\Api\Data\CartItemInterface $cartItem);
+}

app/code/Magento/Quote/Model/Webapi/SaveGuestCartItemWithCartId.php

@@ -0,0 +1,41 @@
+<?php
+/**
+ *
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Quote\Model\Webapi;
+
+use Magento\Quote\Api\Data\CartItemInterface;
+use Magento\Quote\Api\GuestCartItemRepositoryInterface;
+
+/**
+ * GuestCartItemRepository extension to save a cart item for a specific guest cart
+ */
+class SaveGuestCartItemWithCartId implements SaveGuestCartItemWithCartIdInterface
+{
+    /**
+     * @var GuestCartItemRepositoryInterface
+     */
+    protected $guestCartItemRepo;
+
+    /**
+     * Constructor.
+     *
+     * @param GuestCartItemRepositoryInterface $guestCartItemRepo
+     */
+    public function __construct(GuestCartItemRepositoryInterface $guestCartItemRepo)
+    {
+        $this->guestCartItemRepo = $guestCartItemRepo;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function saveForCart($cartId, CartItemInterface $cartItem)
+    {
+        $cartItem->setQuoteId($cartId);
+
+        return $this->guestCartItemRepo->save($cartItem);
+    }
+}

app/code/Magento/Quote/Model/Webapi/SaveGuestCartItemWithCartIdInterface.php

@@ -0,0 +1,24 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Quote\Model\Webapi;
+
+/**
+ * Interface SaveGuestCartItemWithCartIdInterface
+ */
+interface SaveGuestCartItemWithCartIdInterface
+{
+    /**
+     * Add/update the specified cart item for given cartId.
+     *
+     * @param string $cartId The cart ID.
+     * @param \Magento\Quote\Api\Data\CartItemInterface $cartItem The item.
+     * @return \Magento\Quote\Api\Data\CartItemInterface Item.
+     * @throws \Magento\Framework\Exception\NoSuchEntityException The specified cart does not exist.
+     * @throws \Magento\Framework\Exception\CouldNotSaveException The specified item could not be saved to the cart.
+     * @throws \Magento\Framework\Exception\InputException The specified item or cart is not valid.
+     */
+    public function saveForCart($cartId, \Magento\Quote\Api\Data\CartItemInterface $cartItem);
+}

app/code/Magento/Quote/Test/Unit/Model/Webapi/SaveCartItemWithCartIdTest.php

@@ -0,0 +1,74 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+namespace Magento\Quote\Test\Unit\Model\Webapi;
+
+use Magento\Framework\TestFramework\Unit\Helper\ObjectManager;
+use Magento\Quote\Api\CartItemRepositoryInterface;
+use Magento\Quote\Api\Data\CartItemInterface;
+use Magento\Quote\Model\Webapi\SaveCartItemWithCartId;
+
+/**
+ * Test for \Magento\Quote\Model\Webapi\SaveCartItemWithCartId
+ */
+class SaveCartItemWithCartIdTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @var CartItemRepositoryInterface
+     */
+    private $cartItemRepository;
+
+    /**
+     * @var SaveCartItemWithCartId
+     */
+    private $model;
+
+    protected function setUp()
+    {
+        $this->cartItemRepository = $this->getMockBuilder(CartItemRepositoryInterface::class)
+            ->getMockForAbstractClass();
+
+        $this->model = (new ObjectManager($this))->getObject(
+            SaveCartItemWithCartId::class,
+            [
+                'cartItemRepository' => $this->cartItemRepository
+            ]
+        );
+    }
+
+    public function testQuoteIdIsCorrectlySet()
+    {
+        $cartId = 44;
+
+        /** @var CartItemInterface $cartItem */
+        $cartItem = $this->getMockBuilder(CartItemInterface::class)->getMockForAbstractClass();
+        $cartItem->expects($this->once())
+            ->method('setQuoteId')
+            ->with($cartId)
+            ->willReturnSelf();
+
+        $this->model->saveForCart($cartId, $cartItem);
+    }
+
+    public function testQuoteItemIsCorrectlySaved()
+    {
+        $cartId = 44;
+
+        /** @var CartItemInterface $cartItem */
+        $cartItem = $this->getMockBuilder(CartItemInterface::class)->getMockForAbstractClass();
+        $cartItem->expects($this->once())
+            ->method('setQuoteId')
+            ->with($cartId)
+            ->willReturnSelf();
+
+        $this->cartItemRepository->expects($this->once())
+            ->method('save')
+            ->with($cartItem)
+            ->willReturn($cartItem);
+
+        $this->model->saveForCart($cartId, $cartItem);
+    }
+}

app/code/Magento/Quote/Test/Unit/Model/Webapi/SaveGuestCartItemWithCartIdTest.php

@@ -0,0 +1,74 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+namespace Magento\Quote\Test\Unit\Model\Webapi;
+
+use Magento\Framework\TestFramework\Unit\Helper\ObjectManager;
+use Magento\Quote\Api\GuestCartItemRepositoryInterface;
+use Magento\Quote\Api\Data\CartItemInterface;
+use Magento\Quote\Model\Webapi\SaveGuestCartItemWithCartId;
+
+/**
+ * Test for \Magento\Quote\Model\Webapi\SaveGuestCartItemWithCartId
+ */
+class SaveGuestCartItemWithCartIdTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @var GuestCartItemRepositoryInterface
+     */
+    private $guestCartItemRepo;
+
+    /**
+     * @var SaveGuestCartItemWithCartId
+     */
+    private $model;
+
+    protected function setUp()
+    {
+        $this->guestCartItemRepo = $this->getMockBuilder(GuestCartItemRepositoryInterface::class)
+            ->getMockForAbstractClass();
+
+        $this->model = (new ObjectManager($this))->getObject(
+            SaveGuestCartItemWithCartId::class,
+            [
+                'guestCartItemRepo' => $this->guestCartItemRepo
+            ]
+        );
+    }
+
+    public function testQuoteIdIsCorrectlySet()
+    {
+        $cartId = '298047a8923470823';
+
+        /** @var CartItemInterface $cartItem */
+        $cartItem = $this->getMockBuilder(CartItemInterface::class)->getMockForAbstractClass();
+        $cartItem->expects($this->once())
+            ->method('setQuoteId')
+            ->with($cartId)
+            ->willReturnSelf();
+
+        $this->model->saveForCart($cartId, $cartItem);
+    }
+
+    public function testQuoteItemIsCorrectlySaved()
+    {
+        $cartId = '298047a8923470823';
+
+        /** @var CartItemInterface $cartItem */
+        $cartItem = $this->getMockBuilder(CartItemInterface::class)->getMockForAbstractClass();
+        $cartItem->expects($this->once())
+            ->method('setQuoteId')
+            ->with($cartId)
+            ->willReturnSelf();
+
+        $this->guestCartItemRepo->expects($this->once())
+            ->method('save')
+            ->with($cartItem)
+            ->willReturn($cartItem);
+
+        $this->model->saveForCart($cartId, $cartItem);
+    }
+}

app/code/Magento/Quote/etc/di.xml

@@ -15,6 +15,7 @@
     <preference for="Magento\Quote\Api\Data\CartItemInterface" type="Magento\Quote\Model\Quote\Item" />
     <preference for="Magento\Quote\Api\Data\CartInterface" type="Magento\Quote\Model\Quote" />
     <preference for="Magento\Quote\Api\CartItemRepositoryInterface" type="Magento\Quote\Model\Quote\Item\Repository" />
+    <preference for="Magento\Quote\Model\Webapi\SaveCartItemWithCartIdInterface" type="Magento\Quote\Model\Webapi\SaveCartItemWithCartId" />
     <preference for="Magento\Quote\Api\CartRepositoryInterface" type="Magento\Quote\Model\QuoteRepository" />
     <preference for="Magento\Quote\Api\Data\CartSearchResultsInterface" type="Magento\Framework\Api\SearchResults" />
     <preference for="Magento\Quote\Api\PaymentMethodManagementInterface" type="Magento\Quote\Model\PaymentMethodManagement" />
@@ -30,6 +31,7 @@
     <preference for="Magento\Quote\Api\GuestCartManagementInterface" type="Magento\Quote\Model\GuestCart\GuestCartManagement" />
     <preference for="Magento\Quote\Api\GuestCartRepositoryInterface" type="Magento\Quote\Model\GuestCart\GuestCartRepository" />
     <preference for="Magento\Quote\Api\GuestCartItemRepositoryInterface" type="Magento\Quote\Model\GuestCart\GuestCartItemRepository" />
+    <preference for="Magento\Quote\Model\Webapi\SaveGuestCartItemWithCartIdInterface" type="Magento\Quote\Model\Webapi\SaveGuestCartItemWithCartId" />
     <preference for="Magento\Quote\Api\GuestCouponManagementInterface" type="Magento\Quote\Model\GuestCart\GuestCouponManagement" />
     <preference for="Magento\Quote\Api\GuestPaymentMethodManagementInterface" type="Magento\Quote\Model\GuestCart\GuestPaymentMethodManagement" />
     <preference for="Magento\Quote\Api\GuestCartTotalRepositoryInterface" type="Magento\Quote\Model\GuestCart\GuestCartTotalRepository" />

app/code/Magento/Quote/etc/webapi.xml

@@ -176,14 +176,14 @@
             <resource ref="Magento_Cart::manage" />
         </resources>
     </route>
-    <route url="/V1/carts/:quoteId/items" method="POST">
-        <service class="Magento\Quote\Api\CartItemRepositoryInterface" method="save"/>
+    <route url="/V1/carts/:cartId/items" method="POST">
+        <service class="Magento\Quote\Model\Webapi\SaveCartItemWithCartIdInterface" method="saveForCart"/>
         <resources>
             <resource ref="Magento_Cart::manage" />
         </resources>
     </route>
     <route url="/V1/carts/:cartId/items/:itemId" method="PUT">
-        <service class="Magento\Quote\Api\CartItemRepositoryInterface" method="save"/>
+        <service class="Magento\Quote\Model\Webapi\SaveCartItemWithCartIdInterface" method="saveForCart"/>
         <resources>
             <resource ref="Magento_Cart::manage" />
         </resources>
@@ -203,13 +203,13 @@
         </resources>
     </route>
     <route url="/V1/guest-carts/:cartId/items" method="POST">
-        <service class="Magento\Quote\Api\GuestCartItemRepositoryInterface" method="save"/>
+        <service class="Magento\Quote\Model\Webapi\SaveGuestCartItemWithCartIdInterface" method="saveForCart"/>
         <resources>
             <resource ref="anonymous" />
         </resources>
     </route>
     <route url="/V1/guest-carts/:cartId/items/:itemId" method="PUT">
-        <service class="Magento\Quote\Api\GuestCartItemRepositoryInterface" method="save"/>
+        <service class="Magento\Quote\Model\Webapi\SaveGuestCartItemWithCartIdInterface" method="saveForCart"/>
         <resources>
             <resource ref="anonymous" />
         </resources>
@@ -232,7 +232,7 @@
         </data>
     </route>
     <route url="/V1/carts/mine/items" method="POST">
-        <service class="Magento\Quote\Api\CartItemRepositoryInterface" method="save"/>
+        <service class="Magento\Quote\Model\Webapi\SaveCartItemWithCartIdInterface" method="saveForCart"/>
         <resources>
             <resource ref="self" />
         </resources>
@@ -241,7 +241,7 @@
         </data>
     </route>
     <route url="/V1/carts/mine/items/:itemId" method="PUT">
-        <service class="Magento\Quote\Api\CartItemRepositoryInterface" method="save"/>
+        <service class="Magento\Quote\Model\Webapi\SaveCartItemWithCartIdInterface" method="saveForCart"/>
         <resources>
             <resource ref="self" />
         </resources>