Fix #3247.

Fixes creating records in a through association with a polymorphic source
type.

activerecord/CHANGELOG

@@ -8,7 +8,7 @@
     class User < ActiveRecord::Base
       store :settings, accessors: [ :color, :homepage ]
     end
-    
+
     u = User.new(color: 'black', homepage: '37signals.com')
     u.color                          # Accessor stored attribute
     u.settings[:country] = 'Denmark' # Any attribute, even if not specified with an accessor
@@ -37,6 +37,12 @@
 
   [Aaron Christy]
 
+*Rails 3.1.2 (unreleased)*
+
+* Fix creating records in a through association with a polymorphic source type. [GH #3247]
+
+  [Jon Leighton]
+
 *Rails 3.1.1 (October 7, 2011)*
 
 * Add deprecation for the preload_associations method. Fixes #3022.

activerecord/lib/active_record/associations/through_association.rb

@@ -44,7 +44,7 @@ module ActiveRecord
           join_attributes = {
             source_reflection.foreign_key =>
               records.map { |record|
-                record.send(source_reflection.association_primary_key)
+                record.send(source_reflection.association_primary_key(reflection.klass))
               }
           }
 

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -825,4 +825,9 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
   def test_explicitly_joining_join_table
     assert_equal owners(:blackbeard).toys, owners(:blackbeard).toys.with_pet
   end
+
+  def test_has_many_through_with_polymorphic_source
+    post = tags(:general).tagged_posts.create! :title => "foo", :body => "bar"
+    assert_equal [tags(:general)], post.reload.tags
+  end
 end