prevent mass assignment of polymorphic type when using `build`

Closes #8265

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Prevent mass assignment to the type column of polymorphic associations when using `build`
+    Fix #8265
+
+    *Yves Senn*
+
 *   Fix postgresql adapter to handle BC timestamps correctly
 
         HistoryEvent.create!(:name => "something", :occured_at => Date.new(0) - 5.years)

activerecord/lib/active_record/associations/association.rb

@@ -232,7 +232,8 @@ def stale_state
 
         def build_record(attributes)
           reflection.build_association(attributes) do |record|
-            attributes = create_scope.except(*(record.changed - [reflection.foreign_key]))
+            skip_assign = [reflection.foreign_key, reflection.type].compact
+            attributes = create_scope.except(*(record.changed - skip_assign))
             record.assign_attributes(attributes)
           end
         end

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1579,6 +1579,14 @@ def test_abstract_class_with_polymorphic_has_many
     assert_equal [tagging], post.taggings
   end
 
+  def test_build_with_polymotphic_has_many_does_not_allow_to_override_type_and_id
+    welcome = posts(:welcome)
+    tagging = welcome.taggings.build(:taggable_id => 99, :taggable_type => 'ShouldNotChange')
+
+    assert_equal welcome.id, tagging.taggable_id
+    assert_equal 'Post', tagging.taggable_type
+  end
+
   def test_dont_call_save_callbacks_twice_on_has_many
     firm = companies(:first_firm)
     contract = firm.contracts.create!