Fix issue 16315: Product save with onthefly index ignores website assignments

Author: tna274

app/code/Magento/Catalog/Model/Indexer/Product/Flat/Action/Row.php

@@ -85,10 +85,24 @@ public function execute($id = null)
         $ids = [$id];
         $linkField = $this->metadataPool->getMetadata(ProductInterface::class)->getLinkField();
 
+        $select = $this->_connection->select();
+        $select->from(['e' => $this->_productIndexerHelper->getTable('store')], ['e.store_id'])
+            ->where('c.product_id = ' . $id)
+            ->joinLeft(
+                ['c' => $this->_productIndexerHelper->getTable('catalog_product_website')],
+                'e.website_id = c.website_id',
+                []
+            );
+        $storeIds = $this->_connection->fetchCol($select);
+
         $stores = $this->_storeManager->getStores();
         foreach ($stores as $store) {
             $tableExists = $this->_isFlatTableExists($store->getId());
             if ($tableExists) {
+                if (!in_array($store->getId(), $storeIds)) {
+                    $this->flatItemEraser->deleteProductsFromStore($id, $store->getId());
+                    continue;
+                }
                 $this->flatItemEraser->removeDeletedProducts($ids, $store->getId());
                 $this->flatItemEraser->removeDisabledProducts($ids, $store->getId());
             }