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

class DescriptionVersion < ApplicationRecord
  belongs_to :issue
  belongs_to :merge_request

  validate :e
    
  def exactly_one_issuable
    issuable_count = self.class.issuable_attrs.count { |attr| self["#{attr}_id"] }

    if issuable_count != 1
      errors.add(
        :base,
        _("Exactly one of %{attributes} is required") %
          { attributes: self.class.issuable_attrs.join(', ') }
      )
    end