Fix touching an invalid parent record for belongs_to

If the parent of a `belongs_to` record fails to be saved due to
validation errors, `touch` will be called on a new record, which causes
an exception (see https://github.com/rails/rails/pull/9320).

Example:

    class Owner < ActiveRecord::Base
      validates_presence_of :name
    end

    class Pet < ActiveRecord::Base
      belongs_to :owner, touch: true
    end

    pet = Pet.new(owner: Owner.new)

    # Before, this line would raise ActiveRecord::ActiveRecordError
    # "can not touch on a new record object"
    pet.save

1 files changed, 12 insertions, 0 deletions

diff --git a/activerecord/test/cases/timestamp_test.rb b/activerecord/test/cases/timestamp_test.rb
index bb034848e1..777a2b70dd 100644
--- a/activerecord/test/cases/timestamp_test.rb
+++ b/activerecord/test/cases/timestamp_test.rb
@@ -113,6 +113,18 @@ class TimestampTest < ActiveRecord::TestCase
     assert_not_equal previously_owner_updated_at, pet.owner.updated_at
   end
 
+  def test_saving_a_new_record_belonging_to_invalid_parent_with_touch_should_not_raise_exception
+    klass = Class.new(Owner) do
+      def self.name; 'Owner'; end
+      validate { errors.add(:base, :invalid) }
+    end
+
+    pet = Pet.new(owner: klass.new)
+    pet.save!
+
+    assert pet.owner.new_record?
+  end
+
   def test_saving_a_record_with_a_belongs_to_that_specifies_touching_a_specific_attribute_the_parent_should_update_that_attribute
     klass = Class.new(ActiveRecord::Base) do
       def self.name; 'Pet'; end