thinkphp 5.1 db更新问题

$subject_obj = db('subjects', $this->db, false);
...
$res_upd = $subject_obj->where($where)->update($data);
var_dump($where);
var_dump($data);
var_dump($subject_obj->getLastSql());

$subject_obj = db('subjects', $this->db, false);
...
$res_upd = db('subjects', $this->db, false)->where($where)->update($data);
var_dump($where);
var_dump($data);
var_dump($subject_obj->getLastSql());

第一种updat额 里面会把主键id=null