How can I get multiple values in a RESTful endpoint from a field who points to other resource?
I haven't used this module in a while, so I'm not sure if there's a major difference in the approach between 1.x and 2.x. 

I would just implement a custom function for getting the single resource, so that you have full control over what is ret

/*
* Overrides \RestfulEntityBase::controllersInfo().
* Allows us to specify custom functions for different HTTP actions.
*/
public static function controllersInfo() {
  return array(      
   '^.*$' =&gt; array(
     \RestfulInterface::GET =&gt; 'customGetSingleExample',
   ),
  );
}

/**
* Return custom data for getting single resource result 
*/
public function customGetSingleExample() {

  //grab request values
  $request = $this-&gt;getRequest();
  static::cleanRequest($request);

  //do stuff with request (EntityFieldQuery etc)

  .....

  return $result;
}


This is within a custom resource class, which extends RestfulEntityBaseNode

class RestfulCustomResource extends RestfulEntityBaseNode {