Thinkphp 5.1 DB update issue

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

The first type of upDAT will set the primary key ID = NULL