nested relations in ActiveRecord

class State < ActiveRecord::Base
 has_many :cities
end

class City < ActiveRecord::Base
 has_many :streets
 belongs_to :state
end

class Street < ActiveRecord::Base
 has_many :houses
 belongs_to :city
end

class House < ActiveRecord::Base
 belongs_to :street
end

How do you get all of the Houses in virginia?

Well, you could do this:

virginia.cities.collect{|c| c.streets}.flatten.uniq.collect{|s| s.houses}.flatten.uniq

... but that's epically lame. It looks like shit and produces an metric ton of queries and as a result is highly inefficient.

How about this:

House.find(
 :all,
 :include => {:street => {:city => :state}},
 :conditions => {'states.id' => virginia.id}
)

This is a single query with joins where appropriate, and it's a finder on House which is what you're getting anyways. Makes sense, no?

http://joshsharpe.com/archives/56