无法从对象$this变量获取属性

我真的对形势感到困惑。我想从我的类中的函数(由ActiveRecord模型创建)获取数据。这里是一个类:

class Bag extends ActiveRecord\Model {
    static $table_name = "bags";
    static $primary_key = 'bag_id';

    public function get_pocket_types() {
        $arr = json_decode($this->pocket_types);
        return $arr;
    }
}

我把它叫做我的主代码:

$bag = Bag::find_by_bag_id((int)$_GET['id']);
$types = $bag->get_pocket_types();

看起来不错,但我有个错误 Notice: Undefined property: Bag::$pocket_types in models/Bag.php on line 21 当我试图得到 $this->pocket_types .这个字段是绝对存在的,就像一个字段一样 bag_id .

我甚至尝试过调试它(在函数get_pocket_types()中):

echo $this->bag_id;
// OK

echo $this->bag_id_NOT_EXISTS;
// Fatal error: Uncaught exception 'ActiveRecord\UndefinedPropertyException' with message 'Undefined property: Bag->bag_id_NOT_EXISTS
// This is just for catch an error of REALLY not existed field

echo $this->pocket_types;
// Notice: Undefined property: Bag::$pocket_types in models/Bag.php on line 21

我打过电话 var_dump($this); 在函数中:

对象(包)16(6)[“错误”]=>空
[“attributes”:“activerecord\model”:private]=>数组(2){ [“Bag_ID”]=> 利息(160) [“口袋类型”]=> 字符串(0)“[”uu dirty“:”activerecord\model“:private”=>数组(0)[”u readonly“:”activerecord\model“:private”=>
bool(false)[“uu relationships”:“activerecord\model”:private]=>
数组(0)[“uu new_record”:“activerecord\model”:private]=>
布尔(假)

有人能解释一下发生了什么吗?