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: power_of_attorney.rb
    path of file: ./repos/vets-api/modules/claims_api/app/models/claims_api
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

require 'json_marshal/marshaller'
require 'common/file_helpers'

module ClaimsApi
  class PowerOfAttorney < ApplicationRecord
    include FileData
    serialize :auth_headers, JsonMarshal::Marshaller
    serialize :form_data, JsonMarshal::Marshaller
    serialize :source_data, JsonMarshal::Marshaller
    has_kms_key
    has_encrypted :auth_headers, :form_data, :source_
    
    def self.find_using_identifier_and_source(source_name:, id: nil, header_md5: nil, md5: nil)
      primary_identifier = {}
      primary_identifier[:id] = id if id.present?
      primary_identifier[:header_md5] = header_md5 if header_md5.present?
      primary_identifier[:md5] = md5 if md5.present?
      # it's possible to have duplicate POAs, so be sure to return the most recently created match
      poas = ClaimsApi::PowerOfAttorney.where(primary_identifier).order(created_at: :desc)
      poas = poas.select { |poa| poa.source_data['name'] == source_name }
      return nil if poas.blank?

      poas.last
    end