How to get Extension Attribute in a model in Magento 2?

Question

I have added an extension attribute by creating extension_attributes.xml file in etc. Its content is:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
   <extension_attributes for="Magento\Quote\Api\Data\AddressInterface">
      <attribute code="custom_field" type="string" />
   </extension_attributes>
</config>

Now when I try to get extension attribute in model I use following code:

 $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
 $address = $objectManager->get('\Magento\Quote\Api\Data\AddressInterface');
 echo $address->getExtensionAttributes()->getCustomField();

But the problem is $address->getExtensionAttributes() return NULL. What am I doing wrong here?

Answer 1

As also written here (different question but same answer):

The documentation for extension attributes is not quite clear on what's the right way to use them and it seems like each entity type does it in a slightly different way.

That being said, from my experience you have to create plugins to populate the extension attributes, and also to save them. It is not done automatically.

Take a look at Magento\GiftMessage\Model\Plugin\OrderGet and Magento\GiftMessage\Model\Plugin\OrderSave for an example.