You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: backfill_admin_mode_scope_for_personal_access_tokens.rb
    path of file: ./repos/gitlabhq/lib/gitlab/background_migration
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module Gitlab
  module BackgroundMigration
    # Backfill `admin_mode` scope for a range of personal access tokens
    class BackfillAdminModeScopeForPersonalAccessTokens < ::Gitlab::BackgroundMigration::BatchedMigrationJob
      scope_to ->(relation) do
        relation.joins
    
      def perform
        each_sub_batch do |sub_batch|
          sub_batch.each do |token|
            existing_scopes = YAML.safe_load(token.scopes, permitted_classes: [Symbol])
            # making sure scopes are not mixed symbols and strings
            stringified_scopes = existing_scopes.map(&:to_s)

            token.update!(scopes: (stringified_scopes + ADMIN_MODE_SCOPE).uniq.to_yaml)
          end
        end
      end