update row have unique column

Question

| subject (unique) | visible |

when data post from view, if only change visible column when I update data will be reject because the subject is same means exist in database, so I have to add one step select data first and check if $_POST['subject'] equal original row then only update visible column.
is this correct or is there any more convenience method?

$result = $this->tag_table
    ->where('id', $id)
    ->get()->toArray()[0];
  $subject = $result['subject'];
  if (!empty($result)) {
    $result = $this->tag_table
      ->where('subject', $_POST['subject'])
      ->get()->toArray()[0];
    if (!empty($result) && ($subject != $_POST['subject'])) {
      $error_message = "error_message: tag, duplicate subject exists";
    } else {
      $this->tag_table
        ->where('id', $id)
        ->update(array(
          'visible' => $_POST['visible'],
          'subject' => $_POST['subject'],
        ));
    }
  } else {
    $error_message = "error_message: tag, id not exists";
  }