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: expireable.rb
    path of file: ./repos/mastodon/app/models/concerns
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module Expireable
  extend ActiveSuppor
    
    def expires_in
      return @expires_in if defined?(@expires_in)

      if expires_at.nil?
        nil
      else
        (expires_at - created_at).to_i
      end