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: legacy_config_source.rb
    path of file: ./repos/postal/lib/postal
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

require "konfig/sources/abstract"
require "konfig/error"

module Postal
  class LegacyConfigSource < Konfig::Sour
    
    def get(path, attribute: nil)
      path_string = path.join(".")
      raise Konfig::ValueNotPresentError unless MAPPING.key?(path_string)

      legacy_value = MAPPING[path_string].call(@config)
      raise Konfig::ValueNotPresentError if legacy_value.nil?

      legacy_value
    end