Serial joins in a Rails 3 model; how to associate through a many-to-many relationship? -

i have model x , model y associated through has_and_belongs_to_many macro. there's table in database retains data on many-to-many relationships between model x , model y.

i have model a, associated through has_many macro model x. want able run simple command reach model y objects associated model a. so, in other words, let's have following objects:

objecta.kind_of? modela = true objectx.kind_of? modelx = true objecty.kind_of? modely = true 

i want able run objecta.objectys , have returned me [objecty].

what must put in model definitions able this?

(i have tried placing in model a: (1) has_many :modely; , (2) has_many :modely, :through => :modelx. neither right.)

caveat: 1 wouldn't use such perverse model names.

assuming models this:

class modela < activerecord::base   has_many :model_xes end  class modelx < activerecord::base   belongs_to :model_a   has_and_belongs_to_many :model_ies, :class_name => 'modely',      :join_table => 'model_x_model_ies' end  class modely < activerecord::base   has_and_belongs_to_many :model_xes, :class_name => 'modelx',     :join_table => 'model_x_model_ies' end 

we can create scope modely's modelx's belonging model a:

class modely < activerecord::base   scope :find_by_a, lambda { |a| joins(:model_xes).\     where(:model_xes=>{:model_a_id=>a.id})} end 

then simple method call scope on instance of modela:

class modela < activerecord::base   def ys     modely.find_by_a(self)   end end 

test so:

require 'spec_helper'  describe modela   before(:each)     @a = modela.create(:name=>"a")     2.times { @a.model_xes.create(:name=>"x") }   end    "relates model_x"     @a.model_xes.count.should == 2   end    "relates model y"     x = @a.model_xes.first     x.model_ies.create     x.model_ies.count.should == 1   end    "relates model y through model x"     @a.model_xes.each |x|       2.times { x.model_ies.create(:name=>"y") }     end     ys = @a.ys     ys.count.should == 4     ys.all? { |y| y.name == "y" }.should be_true   end end 

note habtm has fallen out of favor, you're encouraged use has_many :through instead.