[rails3] rewrite complicated query with union in arel?

Question

I have the following models

class Courier < ActiveRecord::Base
  has_many :coverages
end

class Coverage < ActiveRecord::Base
  belongs_to :courier
  belongs_to :country_code
end

class CountryCode < ActiveRecord::Base      
end

and then i have the following query:

# could i translate this into cleaner arel?
result = Courier.find_by_sql(<<SQL
          select * from
          (
            select cc.*, cv.rate from couriers cc, coverages cv
            where cv.country_code_id=#{country.id} and cv.courier_id=cc.id
          union
            select cc.*, cv.rate from couriers cc, coverages cv
            where cv.country_code_id is null
              and cv.courier_id=cc.id
              and cv.courier_id not in (select courier_id from coverages where country_code_id=#{country.id})
          ) as foo order by rate asc
SQL
)

In short: i am looking for all couriers that have a coverage for the given country-code or a coverage with an empty country-code instead (the fallback).

The query works, but i am wondering if there are any better ways to write it?