Fatal error: Call to a member function fetch_assoc() on a no

业务场景：使用update语句去更新数据库字段。

原因：update接收值不正确。原来代码：

$query = "UPDATE student SET date = now() WHERE id = $id";
$result = $mysqli->query($query2) or die($mysqli->error); // 问题出现了在这句  
$data = $result->fetch_assoc();

  if ($data) {
      $ary = array('data' => '', 'msg' => '更新成功', 'status' => '0');
  } else {
      $ary = array('data' => '', 'msg' => '更新失败', 'status' => '1');
  }
  exit(JSON($ary));

$query = "UPDATE student SET date = now() WHERE id = $id";
$result = $mysqli->query($query2); // 把or die($mysqli->error)删了
$data = $result->fetch_assoc();
  if ($data) {
      $ary = array('data' => '', 'msg' => '更新成功', 'status' => '0');
  } else {
      $ary = array('data' => '', 'msg' => '更新失败', 'status' => '1');
  }
  exit(JSON($ary));