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: pool_version.rb
    path of file: ./repos/e621ng/app/models
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

class PoolVersion < ApplicationRecord
  user_status_counter :pool_edit_count, foreign_key: :updater_id
  belongs_to :upd
    
  def fill_changes
    if previous
      self.added_post_ids = post_ids - previous.post_ids
      self.removed_post_ids = previous.post_ids - post_ids
    else
      self.added_post_ids = post_ids
      self.removed_post_ids = []
    end