Saving sets of rules in config files - Model Function

So hello everybody, i was wondering if you could save sets of rules in the config file, but having a custom function of a model 

Kinda like combining this :

https://www.codeigniter.com/userguide3/l...onfig-file

PHP Code:

$config = array(
        'signup' => array(
                array(
                        'field' => 'username',
                        'label' => 'Username',
                        'rules' => 'required'
                ),
                array(
                        'field' => 'password',
                        'label' => 'Password',
                        'rules' => 'required'
                ),
                array(
                        'field' => 'passconf',
                        'label' => 'Password Confirmation',
                        'rules' => 'required'
                ),
                array(
                        'field' => 'email',
                        'label' => 'Email',
                        'rules' => 'required'
                )
        )
); 


With this : 

https://www.codeigniter.com/userguide3/l...-as-a-rule

PHP Code:

$this->form_validation->set_rules(
        'username', 'Username',
        array(
                'required',
                array('username_callable', array($this->users_model, 'valid_username'))
        )
); 


I tried with callback , and it worked, but i wanted to use directly the model method.(I tried adding the code but it gives me a error as users_model doesnt exist, and i added the model in the autoload)