Ruby on Rails: Get all entities that have zero occurences on a particular association and condition

Question

I am using Rails 2.3.8

I have two models that represent bikes and rentals on bikes respectively. One bike can have many rentals but at most one "acive" rental. I distinguish the active rentals with a status field in bike_rentals table with value BIKE_RENTAL_OUT (with other two possible values: BIKE_RENTAL_RETURNED and BIKE_RENTAL_INCIDENT)

To create a new rental I want to present to the user a listing of all bikes that are not currently rented, that is all the bikes that have no "active" rental (zero occurrences on an association with a particular condition).

I came up with a solution using a named scope as shown below:

class Bike < ActiveRecord::Base
    has_many :bike_rentals
    named_scope :not_rented,
                :conditions => "bikes.id NOT IN " + 
                     "(SELECT bike_id FROM bike_rentals " + 
                     "WHERE bike_rentals.status = 'BIKE_RENTAL_OUT')"

end

class BikeRental < ActiveRecord::Base
    belongs_to :bike
end

and in the controller I get that list as:

@bikes = Bike.not_rented.find(:all, :order => 'chasis_number')

The code above works, but I got the feeling that it must be a better solution or a more "The rails way" solution for this (presumable common) problem.

I know that I could use a counter cache for the number of "active" rentals a bike has but it seems a too complex solution for the task of just selecting all non "active" rentals which at most will return 1 per bike.

Any ideas?

Answer 1

If I understand correctly, You want to return all bikes which don't have a 'BIKE_RENTAL_OUT' status in the bikes has_may associated table bike_rentals. This should be able to be scoped like this:

named_scope :not_rented, :joins => :bike_rentals, :conditions => [ "bike_rentals.status != ?", BIKE_RENTAL_OUT]

Hope this helps