Code to Db-Connected Apigility - How Extend Collection Class and get only necessary fields

Question

I follow this tutorial http://techportal.inviqa.com/2013/12/03/create-a-restful-api-with-apigility/ and create a db-connected API from code-connected.

And have for Entity the above methods that get me the possibility to choice what field are returned in JSON

public function getArrayCopy()
{
    return array(
        'id' => $this->id,
        'cat_id' => $this->cat_id,
        'name' => $this->name,
        'url' => $this->url,
    );
}

public function exchangeArray(array $array)
{
    $this->id = $array['id'];
    $this->cat_id = $array['cat_id'];
    $this->name = $array['name'];
    $this->url = $array['url'];
}

Now i wont do the same thing for Collections (return only some fields of all table field in db)... but can't understood where.

I have the Resource that call Mapper

return $this->mapper->fetchAll();

and in mapper

public function fetchAll()
{
    $select = new Select('eye_medic');
    $paginatorAdapter = new DbSelect($select, $this->adapter);
    $collection = new MedicCollection($paginatorAdapter);
    return $collection;
}

how i can edit this for create my own collection arrays and get only chosed by me fields?

In example Response for entity (very good):

[id] => 3
[cat_id] => 1
[name] => Abbadini Dr. Pasqualino Odontoiatra 
[url] => abbadini-dr-pasqualino-odontoiatra
[_links] => stdClass Object ...

Response for collection are with full database field (not good, i want only 4 fields)

[15] => stdClass Object
 (
   [id] => 19
   [user_id] => 0
   [country_id] => 105
   [region_id] => 13
   [province_id] => 68
   [city_id] => 5889
   [url] => amicarelli-dr-alfonso-studio-dentistico
   [name] => Amicarelli Dr. Alfonso Studio Dentistico 
   [description] => 
   [poster] => 
   [cat_id] => 1
   [status] => LIVE
   [publish_ts] => 2014-11-13 18:46:47
   [edited_ts] => 
   [_links] => stdClass Object ...

Thanks in advance