Extending existing user model

Hello
I have a problem with extending the method of existing model. 
Here is the example. The user model located in the file: /application/model/subscriptions_model.php :

Code:

class Subscriptions_model extends App_Model {
    public function __construct() {
        parent::__construct();
    }
    public function get_child_invoices($id) {
       return $child;
    }}

I'd like to extend method of the model in the new file `/application/model/my_subscriptions_model.php`.

Code:

class My_subscriptions_model extends Subscriptions_model {
    public function __construct() {
        parent::__construct();
    }
    public get_child_invoices($id) {
        return $child;
    }}

The goal is to rewrite existing method get_child_invoices  of  Subscriptions_model .
Is it possible to resolve this issue?
Or as a way to expand the subscription controller Subscriptions (/application/controllers/subscription.php) and call the another module.
Thank you.

If you want to extend the CodeIgniter model, it's CI_Model not app_model.

What did you Try? What did you Get? What did you Expect?

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