Rails Active Record CrossTab查询

Question

A vehicle has_many services. services has_many types (0,1,2,3,4).

I need to write a query that shows the mileage reading of when each type of service was last completed for a vehicle.

In my MS ACCESS days this was a simple crosstab (transform) query. Could someone point me in the right direction in the rails environment? Thanks

This is the long hand version of what I'm trying to collate:

        @a_last = v.services.maximum(:mileage_closed, :conditions => ["closed = TRUE AND service_type_id > 0"])
        @b_last = v.services.maximum(:mileage_closed, :conditions => ["closed = TRUE AND service_type_id > 1"])
        @c_last = v.services.maximum(:mileage_closed, :conditions => ["closed = TRUE AND service_type_id > 2"])
        @d_last = v.services.maximum(:mileage_closed, :conditions => ["closed = TRUE AND service_type_id > 3"])
        @e_last = v.services.maximum(:mileage_closed, :conditions => ["closed = TRUE AND service_type_id > 4"])

Thanks!

Answer 1

The simple answer is to iterate through a range and collect your results:

mileages = (0..4).collect do |service_type|
  vehicle.services.maximum(:mileage_closed, :conditions => ["CLOSED = TRUE AND SERVICE_TYPE_ID = #{service_type}"]
end

I'm guessing you mean to search the service type id with an equals instead of a more than.

This will, however, make 5 sql statements when it is run. You'll probably find this doesn't effect performance too badly. If performance does become an issue, a different approach would be to write an SQL statement to find your results (it'll be grouped by vehicle_id and service_type_id, then run a MAX() on the mileage_closed column) and make that a view in your database. Then you should be able to find these results in a single database round trip.