Set sortable admin column with ACF field

<?php
function admin_columns($columns) {
  $columns = array(
    'cb'	 	=> '<input type="checkbox" />',
    'title' => '標題',
    'user' => '會員',
    'answser' => '選項',
    'date' => '日期'
  );
  return $columns;
}
function admin_columns_values($column) {
  global $post;
  switch ($column) {
    case 'user':
      echo get_field('pollresult_user')['display_name'];
      break;
    case 'date':
      echo get_the_date('Y / m / d');
      break;
    default:
      # code...
      break;
  }
}
function admin_columns_sortable( $columns ){
  $columns['user'] = '會員';
  return $columns;
}
add_filter("manage_edit-pollresult_columns", "admin_columns");
add_action("manage_pollresult_posts_custom_column", "admin_columns_values");
add_filter("manage_edit-pollresult_sortable_columns", "admin_columns_sortable" );