Fix #3247.

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

Conflicts:

	activerecord/CHANGELOG

activerecord/CHANGELOG

@@ -1,3 +1,9 @@
+*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)*
 
 * Raise an exception if the primary key of a model in an association is needed

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

@@ -820,4 +820,9 @@ 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