Correct flat indexing with staging and > 500 cats.

Previously, the array was prefilled with entity_id values to ensure those categories
were indexed.  Unfortunately, when using row_id, it was using entity_ids.

When chunking at 500, this could cause an overlapping row to be processed twice (the
second time with missing non-static attributes), and cause a duplicate key error.

Author: toddbc

app/code/Magento/Catalog/Model/Indexer/Category/Flat/AbstractAction.php

@@ -369,9 +369,11 @@ protected function getAttributeValues($entityIds, $storeId)
         }
         $values = [];
 
-        foreach ($entityIds as $entityId) {
-            $values[$entityId] = [];
+        $linkIds = $this->getLinkIds($entityIds);
+        foreach ($linkIds as $linkId) {
+            $values[$linkId] = [];
         }
+
         $attributes = $this->getAttributes();
         $attributesType = ['varchar', 'int', 'decimal', 'text', 'datetime'];
         foreach ($attributesType as $type) {
@@ -385,9 +387,36 @@ protected function getAttributeValues($entityIds, $storeId)
                 }
             }
         }
+
         return $values;
     }
 
+    /**
+     * Translate entity ids into link ids
+     *
+     * Used for rows with no EAV attributes set.
+     *
+     * @param array $entityIds
+     * @return array
+     */
+    private function getLinkIds(array $entityIds)
+    {
+        $linkField = $this->getCategoryMetadata()->getLinkField();
+        if ($linkField === 'entity_id') {
+            return $entityIds;
+        }
+
+        $select = $this->connection->select()->from(
+            ['e' => $this->connection->getTableName($this->getTableName('catalog_category_entity'))],
+            [$linkField]
+        )->where(
+            'e.entity_id IN (?)',
+            $entityIds
+        );
+
+        return $this->connection->fetchCol($select);
+    }
+
     /**
      * Return attribute values for given entities and store of specific attribute type
      *