How to call a model from a sub directory in the admin dashboard

Sigal

I can't find the way to call a model in a sub folder in the models folder from the fuel admin dashboard.

I have a sub folder in my application/models folder called: classifieds.

In this folder I have ad_categories_model.php file.

In MY_Fuel_Modules I've tried a few things:
$config['modules']['ad_categories'] = array();
Got an error:
Unable to locate the file: ad_categories_model.php

$config['modules']['ad_categories'] = array('model_location'=>'classifieds');
Got an error:
Unable to locate the file: ad_categories_model.php

$config['modules']['classifieds/ad_categories'] = array('module_name'=>'Ad Categories');
Got this error:

A PHP Error was encountered

Severity: Notice

Message: Undefined property: Module::$model_name

Filename: controllers/module.php

Line Number: 79
An Error Was Encountered

Unable to locate the file: .php

$config['modules']['ad_categories'] = array('module_name'=>'Ad Categories', 'module_uri'=>'classifieds/');

An Error Was Encountered

Unable to locate the file: ad_categories_model.php



Is there no way to have a model in a sub folder?