What is the process for saving an extension attribute to the database, when passed over using the REST API? How does it know which table / field to save the value to? the method are being generated in the OrderExtension.php but don't seem to do anything.
I can't seem to find an example of this that doesn't use the object manager (How to insert data in custom table using REST API in Magento2). I have also looked through the Magento/Giftmessage code, but it seems to just magically know where to save this.
I have tried adding these sort of lines to the extension_attributes.xml -
<join reference_table="" reference_field="" join_on_field=""> <field></field> </join>
But it doesn't seem to do anything when the correct fields are added.