diff --git a/activerecord/lib/active_record/associations/association_proxy.rb b/activerecord/lib/active_record/associations/association_proxy.rb
index 59f1d3b867..676c4ace61 100644
--- a/activerecord/lib/active_record/associations/association_proxy.rb
+++ b/activerecord/lib/active_record/associations/association_proxy.rb
@@ -180,7 +180,10 @@ module ActiveRecord
             record["#{@reflection.options[:as]}_id"]   = @owner.id unless @owner.new_record?
             record["#{@reflection.options[:as]}_type"] = @owner.class.base_class.name.to_s
           else
-            record[@reflection.primary_key_name] = @owner.id unless @owner.new_record?
+            unless @owner.new_record?
+              primary_key = @reflection.options[:primary_key] || :id
+              record[@reflection.primary_key_name] = @owner.send(primary_key)
+            end
           end
         end
 
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index 20b9acda44..a5ae5cd8ec 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -1115,5 +1115,11 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert !client_association.respond_to?(:private_method)
     assert client_association.respond_to?(:private_method, true)
   end
+
+  def test_creating_using_primary_key
+    firm = Firm.find(:first)
+    client = firm.clients_using_primary_key.create!(:name => 'test')
+    assert_equal firm.name, client.firm_name
+  end
 end