MAGETWO-55126: [Backport] [API] Can not link simple product to configurable one - for 2.1.x

 - added concrete implementation of setAssociatedProductIds

Author: cspruiell

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

@@ -22,7 +22,6 @@
  * @method Product setHasError(bool $value)
  * @method \Magento\Catalog\Model\ResourceModel\Product getResource()
  * @method null|bool getHasError()
- * @method Product setAssociatedProductIds(array $productIds)
  * @method array getAssociatedProductIds()
  * @method Product setNewVariationsAttributeSetId(int $value)
  * @method int getNewVariationsAttributeSetId()
@@ -2609,4 +2608,13 @@ private function getMediaGalleryProcessor()
         }
         return $this->mediaGalleryProcessor;
     }
+
+    /**
+     * Set the associated products
+     * @param array $productIds
+     */
+    public function setAssociatedProductIds(array $productIds)
+    {
+        $this->getExtensionAttributes()->setConfigurableProductLinks($productIds);
+    }
 }

app/code/Magento/ConfigurableProduct/Model/LinkManagement.php

@@ -132,7 +132,7 @@ public function removeChild($sku, $childSku)
         if (count($options) == count($ids)) {
             throw new NoSuchEntityException(__('Requested option doesn\'t exist'));
         }
-        $product->addData(['associated_product_ids' => $ids]);
+        $product->setAssociatedProductIds($ids);
         $product->save();
         return true;
     }