diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index 8553211b54..00f7dd6ca2 100644
--- a/activerecord/CHANGELOG
+++ b/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
diff --git a/activerecord/lib/active_record/associations/through_association.rb b/activerecord/lib/active_record/associations/through_association.rb
index b347a94978..f95e5337c2 100644
--- a/activerecord/lib/active_record/associations/through_association.rb
+++ b/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))
               }
           }
 
diff --git a/activerecord/test/cases/associations/has_many_through_associations_test.rb b/activerecord/test/cases/associations/has_many_through_associations_test.rb
index 244e0b174d..cfc0123d9a 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/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