allowedFields in class

CodeIgniter 3 didn't really have a model, it only contained the following:

PHP Code:

class CI_Model {

    /**
     * Class constructor
     *
     * @link    https://github.com/bcit-ci/CodeIgniter/issues/5332
     * @return    void
     */
    public function __construct() {}

    /**
     * __get magic
     *
     * Allows models to access CI's loaded classes using the same
     * syntax as controllers.
     *
     * @param    string    $key
     */
    public function __get($key)
    {
        // Debugging note:
        //    If you're here because you're getting an error message
        //    saying 'Undefined Property: system/core/Model.php', it's
        //    most likely a typo in your model code.
        return get_instance()->$key;
    }

} 


If you wan't the "not a model approach" you can simply use Manual Model Creation.

You can however turn that feature off if you like:
https://codeigniter.com/user_guide/model...ing-fields

PHP Code:

$model->protect(false)
      ->insert($data)
      ->protect(true);