adding a test to demonstrate how to use STI subclasses on the far right

side of a hm:t association along with preloading.

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -37,6 +37,13 @@ def setup
     Reader.create :person_id => 0, :post_id => 0
   end
 
+  def test_preload_sti_rhs_class
+    developers = Developer.includes(:firms).all.to_a
+    assert_no_queries do
+      developers.each { |d| d.firms }
+    end
+  end
+
   def test_preload_sti_middle_relation
     club = Club.create!(name: 'Aaron cool banana club')
     member1 = Member.create!(name: 'Aaron')

activerecord/test/models/contract.rb

 class Contract < ActiveRecord::Base
   belongs_to :company
   belongs_to :developer
+  belongs_to :firm, :foreign_key => 'company_id'
 
   before_save :hi
   after_save :bye

activerecord/test/models/developer.rb

@@ -38,6 +38,8 @@ def find_least_recent
   has_and_belongs_to_many :special_projects, :join_table => 'developers_projects', :association_foreign_key => 'project_id'
 
   has_many :audit_logs
+  has_many :contracts
+  has_many :firms, :through => :contracts, :source => :firm
 
   scope :jamises, -> { where(:name => 'Jamis') }