Drupal Services - Direct file URL instead of file resource reference

I am using Drupal services and services_entity module to build a web service. The problem is that when a file is attached to an entity using fields, etc, the service endpoints display the file as resource reference as:

array (
    resource: file,
    id: xx,
    uri: /entity_file/xx.json
)

The thing is, every time you wish to display a file you will have to make 2 or more requests:

First, get the file entity URI
Second, retrieve the details of the file entity by id to get the direct url for the file (which can be embedded into the application or used as src="xx" for an img tag.

The question is, how to get the file URLs directly without having to make additional requests. So, the preferred response would be:

array (
    resource: file,
    id: xx,
    uri: /entity_file/xx.json,
    url: http://.../sites/.../files/foo/bar/b-reft.jpg
)

I looked for hours but did not find an answer, so I thought I would share the solution I found. I believe it would help many (and I also wish I could share my module for complex index query parameter support for services_entity module).

Solution

Declare a Resource Controller

Since the data is returned by a ServicesEntityResourceController, I decided to declare my own resource controller using hook_services_entity_resource_info().

/**
 * Implements hook_entity_resource_info()
 */
function c11n_services_entity_resource_info() {

    $output = array();

    $output['c11n'] = array (
        'title' => 'Clean Entity Processor - Customized',
        'description' => 'An entity wrapper based on the "Clean Entity Wrapper" wrapper with certain fixes and improvements.',
        'class' => 'ServicesEntityResourceControllerC11n',
    );

    return $output;

}

Declare Controller Class

After this, I declared the controller class:

ServicesEntityResourceControllerC11n extends ServicesEntityResourceControllerClean

Override the get_resource_reference() method

The final touch (toque final) would be to add the file URL. I decided to work on the output of the parent class and add in the URL for a file. Actual data is returned by the ServicesEntityResourceController::get_resource_reference() method. So, I overrode it like this and it was done.

protected function get_resource_reference($resource, $id) {

    $output = parent::get_resource_reference($resource, $id);

    switch ($resource):
        case 'file':
            $file = file_load($id);
            if ($file)
                $output['url'] = file_create_url($file->uri);
            break;
        case 'taxonomy_term':
            // Do something for taxonomy terms
            break;
    endswitch;

    return $output;

}

It solves the issue. However, I do not claim it to be the best solution, but having some solution is better than having none.

Alternative Solution

You can alter the entity_file resource and add a targeted_action named download or embed. In the callback, simply send out the headers for the file mime-type and then render the file contents using fpasthru() or echo file_get_contents().