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: mongoid_extension.rb
    path of file: ./repos/stratify/lib/stratify
    
    the code of the file until where you have to start completion: module Stratify
  module MongoidExtension
    module NaturalKey
      module ClassMethods
        attr_reader :nat
    
        def natural_key_hash
          {}.tap do |hash|
            self.class.natural_key_fields.each do |field|
              hash[field] = self.send(field)
            end
          end
        end