nested relations in ActiveRecord

13 November 2009 // Filed under active record

I personally don’t think this is all that exciting — but I see this question asked a lot and just want something to point at from now on.

Say you have several ‘nested’ has_many relations:

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?

2009-11-13  ::  admin