class Quickbooks::Service::ExchangeRate

def fetch_by_currency(source_currency_code, as_of_date = nil)
  url = url_for_resource(model::REST_RESOURCE)
  params = { sourcecurrencycode: source_currency_code }
  params[:asofdate] = as_of_date unless as_of_date.nil?
  response = do_http_get(url, params)
  if response.code.to_i == 200
    model.from_xml(parse_singular_entity_response(model, response.plain_body))
  else
    nil
  end
end

def model
  Quickbooks::Model::ExchangeRate
end