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: 20190402024053_add_first_unread_at_to_user_stats.rb
    path of file: ./repos/discourse/db/migrate
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

class AddFirstUnreadAtToUserStats < ActiveRecord::Migration[5.2]
  disable_ddl_transaction!

  def up
    # so we can rerun this if the index creation fails out of
    
  def up
    # so we can rerun this if the index creation fails out of ddl
    if !column_exists?(:user_stats, :first_unread_at)
      add_column :user_stats,
                 :first_unread_at,
                 :datetime,
                 null: false,
                 default: -> { "CURRENT_TIMESTAMP" }
    end