added :piggyback option to has_many :through relationships to pick up values from the join table as needed

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@3323 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activerecord/CHANGELOG

@@ -1,5 +1,7 @@
 *SVN*
 
+* added :piggyback option to has_many :through relationships to pick up values from the join table as needed [Tobias Luetke] 
+
 * Fix typo in association docs. #3296. [Blair Zajac]
 
 * Fixed :through relations when using STI inherited classes would use the inherited class's name as foreign key on the join model [Tobias Luetke] 

activerecord/lib/active_record/associations.rb

@@ -881,7 +881,7 @@ def create_has_many_reflection(association_id, options, &extension)
             :class_name, :table_name, :foreign_key,
             :exclusively_dependent, :dependent,
             :select, :conditions, :include, :order, :group, :limit, :offset,
-            :as, :through,
+            :as, :through, :piggyback, 
             :finder_sql, :counter_sql, 
             :before_add, :after_add, :before_remove, :after_remove, 
             :extend

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -39,7 +39,7 @@ def method_missing(method, *args, &block)
             
         def find_target
           @reflection.klass.find(:all, 
-            :select     => "#{@reflection.table_name}.*",
+            :select     => construct_select,
             :conditions => construct_conditions,
             :from       => construct_from,
             :order      => @reflection.options[:order], 
@@ -73,6 +73,14 @@ def construct_from
           "#{@reflection.table_name}, #{@owner.class.reflections[@reflection.options[:through]].table_name}"
         end
         
+        def construct_select
+          selected = ["#{@reflection.table_name}.*"]          
+          if @reflection.options[:piggyback]
+            selected += [@reflection.options[:piggyback]].flatten.collect { |field| "#{@owner.class.reflections[@reflection.options[:through]].table_name}.#{field}" } 
+          end
+          selected.join(', ')
+        end
+        
         def construct_scope
           {
             :find   => { :conditions => construct_conditions },

activerecord/test/associations_join_model_test.rb

@@ -38,5 +38,9 @@ def test_polymorphic_has_many_going_through_join_model
   def test_polymorphic_has_many_going_through_join_model_with_inheritance
     assert_equal tags(:general), posts(:thinking).tags.first
   end
+  
+  def test_has_many_with_piggyback
+    assert_equal "2", categories(:sti_test).authors.first.post_id
+  end
     
 end

activerecord/test/fixtures/category.rb

@@ -6,7 +6,7 @@ def self.what_are_you
   end
   
   has_many :categorizations
-  has_many :authors, :through => :categorizations  
+  has_many :authors, :through => :categorizations, :piggyback => [:post_id]  
 end
 
 class SpecialCategory < Category