Saving an EAV Product attribute via the extensions mechanism

ext_eav.php

<?php

/** @var \Magento\Catalog\Api\ProductRepositoryInterface $productRepo */
$productRepo = $objManager->get(\Magento\Catalog\Api\ProductRepositoryInterface::class);

/** @var \Magento\Catalog\Api\Data\ProductInterface $productData */
$product = $productRepo->get('some_sku');

/** @var \Magento\Catalog\Api\Data\ProductExtensionInterface $ext */
$ext = $product->getExtensionAttributes();

/**
 * At this point, $ext has only "stock_item" as an extension attribute, 
 * because it was added manually on load: http://git.io/vW0xh
 * 
 * "stock_item" seems to be a complex, extension attribute.
 * That is - it does not exist as an EAV attribute.
 * It's added here: http://git.io/vW0xD
 * 
 * Because I fancy auto-completion, I created an extension attribute - "foo_bar" - 
 * mapped to an actual EAV product attribute (aka created via "addAttribute()"):
 * 
 * // somewhere, in an "extension_attributes.xml" file
 * <extension_attributes for="Magento\Catalog\Api\Data\ProductInterface">
 *     <attribute code="foo_bar" type="int" />
 * </extension_attributes>
 */
 
// normally, we would do $product->setCustomAttribute('foo_bar', 3);
$ext->setFooBar(3); // we get auto-completion here. nice!

// do we always have to do this?
$product->setExtensionAttributes($ext);

// this actually saves my EAV attribute correctly, depending on the store I'm in
$productRepo->save($product);

The Product repository being the one converting our extension attributes to the $data array (same as doing $productModel->setData($data)).

So I guess any logic outside the Repository object won't have this behaviour by default.

hi nevvermind, why the CustomAttributesDataInterface extends ExtensibleDataInterface, CUSTOMER ATTRIBUTE is sub of Extension attributes?

@LifeAsBook - yeah, I guess. Why? Also, it's customer attribute data, which is extensible.