MC-18478: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails

Author: rganin

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

@@ -90,7 +90,7 @@ public function addProduct($product)
         $this->_addVisitorToItem($item);
         $item->loadByProduct($product);
 
-        if (!$item->getId()) {
+        if (!$item->getId() && $item->getProductId()) {
             $item->addProductData($product);
             $item->save();
         }

dev/tests/integration/testsuite/Magento/Catalog/Model/Product/Compare/ListCompareTest.php

@@ -44,15 +44,25 @@ protected function tearDown()
         $this->_session->setCustomerId(null);
     }
 
+    /**
+     * @magentoDataFixture Magento/Catalog/_files/second_product_simple.php
+     */
     public function testAddProductWithSession()
     {
         $this->_session->setCustomerId(1);
         /** @var $product \Magento\Catalog\Model\Product */
         $product = \Magento\TestFramework\Helper\Bootstrap::getObjectManager()
             ->create(\Magento\Catalog\Model\Product::class)
             ->load(1);
-        $this->_model->addProduct($product);
+        /** @var $product2 \Magento\Catalog\Model\Product */
+        $product2 = \Magento\TestFramework\Helper\Bootstrap::getObjectManager()
+            ->create(\Magento\Catalog\Model\Product::class)
+            ->load(6);
+        $this->_model->addProducts([$product->getId(), $product2->getId(), 'none', 99]);
         $this->assertTrue($this->_model->hasItems(1, $this->_visitor->getId()));
+        $this->assertTrue($this->_model->hasItems(6, $this->_visitor->getId()));
+        $this->assertFalse($this->_model->hasItems('none', $this->_visitor->getId()));
+        $this->assertFalse($this->_model->hasItems(99, $this->_visitor->getId()));
     }
 
     public function testAddProductWithoutSession()