Join extension attributes are not added to Order and Order Items

[k4emic]

Preconditions

1. Magento 2.1.3
2. 7.0.25-0ubuntu0.16.04.1

Steps to reproduce

1. Define a Extension attribute for the relevant interfaces with a join table:

    <extension_attributes for="Magento\Sales\Api\Data\OrderItemInterface">
        <attribute code="some_id" type="string">
            <join reference_table="SOMETABLE" join_on_field="item_id" reference_field="item_id">
                <field>some_id</field>
            </join>
        </attribute>
    </extension_attributes>
    <extension_attributes for="Magento\Sales\Api\Data\OrderInterface">
        <attribute code="some_id" type="string">
            <join reference_table="SOMETABLE" join_on_field="quote_id" reference_field="quote_id">
                <field>some_id</field>
            </join>
        </attribute>
    </extension_attributes>

2. Make sure there's matching data in the SOMETABLE table and the sales_order_item table
3. Request order(s) or order items via REST

Expected result

1. The "some_id" attribute should be included in the response for orders and order items

Actual result

1. The extension attribute is not included in any of the two responses

This was partially fixed in #8035, but only for order search results

[k4emic]

I've fixed this (for order items) for my project by adding the following patch to our project, until a permanent solution is found. This will only fix the problem for some requests, but might provide a starting point for someone looking for a better fix.

--- Model/Order.php      2018-04-10 17:03:57.092710675 +0200
+++ Model/Order.php	2018-04-10 17:04:39.608326985 +0200
@@ -266,6 +266,10 @@
      * @var \Magento\Framework\Stdlib\DateTime\TimezoneInterface
      */
     protected $timezone;
+    /**
+     * @var \Magento\Framework\Api\ExtensionAttribute\JoinProcessorInterface
+     */
+    private $joinProcessor;

     /**
      * @param \Magento\Framework\Model\Context $context
@@ -322,6 +326,7 @@
         \Magento\Sales\Model\ResourceModel\Order\CollectionFactory $salesOrderCollectionFactory,
         PriceCurrencyInterface $priceCurrency,
         \Magento\Catalog\Model\ResourceModel\Product\CollectionFactory $productListFactory,
+        \Magento\Framework\Api\ExtensionAttribute\JoinProcessorInterface $joinProcessor,
         \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
         \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
         array $data = []
@@ -346,6 +351,7 @@
         $this->_trackCollectionFactory = $trackCollectionFactory;
         $this->salesOrderCollectionFactory = $salesOrderCollectionFactory;
         $this->priceCurrency = $priceCurrency;
+        $this->joinProcessor = $joinProcessor;
         parent::__construct(
             $context,
             $registry,
@@ -1248,6 +1254,7 @@
     public function getItemsCollection($filterByTypes = [], $nonChildrenOnly = false)
     {
         $collection = $this->_orderItemCollectionFactory->create()->setOrderFilter($this);
+        $this->joinProcessor->process($collection);

         if ($filterByTypes) {
             $collection->filterByTypes($filterByTypes);

Hi @k4emic , thank you for your report.
We've acknowledged the issue and added to our backlog.