#20481 REST products update category_ids cannot be removed

Code review, updated unit tests

Author: ygyryn

app/code/Magento/Catalog/Model/ProductRepository.php

@@ -193,7 +193,7 @@ class ProductRepository implements \Magento\Catalog\Api\ProductRepositoryInterfa
      * @param \Magento\Framework\Serialize\Serializer\Json|null $serializer
      * @param int $cacheLimit [optional]
      * @param ReadExtensions|null $readExtensions
-     * @param Magento\Catalog\Api\CategoryLinkManagementInterface|null $categoryLinkManagement
+     * @param Magento\Catalog\Api\CategoryLinkManagementInterface|null $linkManagement
      * @SuppressWarnings(PHPMD.ExcessiveParameterList)
      * @SuppressWarnings(PHPMD.UnusedFormalParameter)
      */
@@ -638,7 +638,7 @@ public function save(ProductInterface $product, $saveOptions = false)
         }
 
         $this->saveProduct($product);
-        if ($assignToCategories === true) {
+        if ($assignToCategories === true && $product->getCategoryIds()) {
             $this->linkManagement->assignProductToCategories(
                 $product->getSku(),
                 $product->getCategoryIds()

app/code/Magento/Catalog/Test/Unit/Model/ProductRepositoryTest.php

@@ -200,7 +200,8 @@ protected function setUp()
                 'setData',
                 'getStoreId',
                 'getMediaGalleryEntries',
-                'getExtensionAttributes'
+                'getExtensionAttributes',
+                'getCategoryIds'
             ]
         );
 
@@ -220,7 +221,8 @@ protected function setUp()
                 'validate',
                 'save',
                 'getMediaGalleryEntries',
-                'getExtensionAttributes'
+                'getExtensionAttributes',
+                'getCategoryIds'
             ]
         );
         $this->initializedProduct->expects($this->any())
@@ -269,6 +271,12 @@ protected function setUp()
         $this->initializedProduct
             ->method('getExtensionAttributes')
             ->willReturn($this->productExtension);
+        $this->product
+            ->method('getCategoryIds')
+            ->willReturn([1, 2, 3, 4]);
+        $this->initializedProduct
+            ->method('getCategoryIds')
+            ->willReturn([1, 2, 3, 4]);
         $storeMock = $this->getMockBuilder(StoreInterface::class)
             ->disableOriginalConstructor()
             ->setMethods([])