rewrite complicated query with union in arel?

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).

开发者_Go百科

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

---

If you wanted to keep the find_by_sql you could condense the query into:

result = Courier.find_by_sql [
  "SELECT cc.*, cv.rate
  FROM couriers cc inner join coverages cv on cv.courier_id = cc.id
  WHERE cv.country_code_id = ?
    OR (cv.country_code_id is null AND cv.courier_id NOT IN (SELECT courier_id FROM coverages WHERE country_code_id= ? ))
  ORDER BY cv.rate asc", country.id, country.id ]

---

With arel it seems not very difficult to get something similar:

country_code = ...
c=Courier.arel_table
cv=Coverage.arel_table    
courier_ids_with_country_code= Coverage.select(:courier_id).where(:country_code=>country_code)
coverage_ids_and_condition= Coverage.select(:id)
                                    .where(cv[:country_code]
                                               .eq(nil)
                                               .and(cv[:courier_id]
                                                          .in(courier_ids_with_country_code)))
coverage_ids_with_country_code= Coverage.select(:id)
                                        .where(:country_code=>country_code)

coverage_union_joined_with_couriers = Coverage.include(:courier)
                                              .where(cv[:id]
                                                     .in(coverage_ids_with_country_code
                                                         .union(coverage_ids_and_condition)))

This will perform one query that gets the coverages and associated couriers for the given conditions. I do not believe it would be very difficult to adapt this in order to get the intended result.