Example.php

<?php

class Example extends CI_Model
{

    const TABLE = 'example';
    private $CI = null;
    private $data = [];

    function __construct($id = null)
    {
        parent::__construct();

        $this->CI =& get_instance();

        if($id) {
            $this->set_id($id);
        }
    }

    function set_id($id)
    {
        if($id) {
            $this->data = $this->CI->s->from(self::TABLE)->where(array('id' => $id))->one();
        }
    }

    static function by_id($id)
    {
        return new Example($id);
    }

    public function __get($name)
    {
        if (isset($this->data[$name])) {
            return $this->data[$name];
        }

        return FALSE;
    }

    public function __set($name, $value)
    {
        $this->data[$name] = $value;
    }

    public function save()
    {
        if(isset($this->data['id'])){
            $this->CI->s->from(self::TABLE)->where(array('id' => $this->id))->update($this->data)->execute();
        }
    }

    public function other_method()
    {
        // code..
    }
}

$class = Example::by_id(1);
$class->status = 'success';
$class->save();