How to rename a Paperclip attachment when an interpolated attribute in the Filename changes

image.rb

class Image < ActiveRecord::Base
  
  attr_accessible :custom_file_name

  has_attached_file :attachment
  
  after_save :rename_attached_file, if: :rename_necessary?

  def rename_attached_file
    (attachment.styles.keys).each do |style|
      new_path = attachment.path(style)
      old_path = new_path.gsub("attachments/#{custom_file_name}", "attachments/#{custom_file_name_was}")
      rename_attachment(attachment, old_path, new_path)
    end
  end
  
  private
  
  def rename_necessary?
    custom_file_name_changed? && !attachment_updated_at_changed?
  end
  
  def rename_attachment(attachment, old_path, new_path)
    case attachment.options[:storage]
    when :filesystem
      File.rename(old_path, new_path)
    when :s3
      attachment.s3_bucket.objects[old_path].move_to(new_path, acl: :public_read)
    end
  end
  
end