polymorphic belongs_to association with touch: true updates old record correctly

Example: Given you have a comments model with a polymorphic commentable
association (e.g. books and songs) with the touch option set.
Every time you update a comment its commentable should be touched.
This was working when you changed attributes on the comment or when you
moved the comment from one book to another. However, it was not working
when moving a comment from a book to a song. This is now fixed.

3 files changed, 63 insertions, 17 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 951ec17a85..ed076c04bb 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,17 @@
+*   Polymorphic belongs_to associations with the `touch: true` option set update the timestamps of
+    the old and new owner correctly when moved between owners of different types.
+
+    Example:
+
+        class Rating < ActiveRecord::Base
+          belongs_to :rateable, polymorphic: true, touch: true
+        end
+
+        rating = Rating.create rateable: Song.find(1)
+        rating.update_attributes rateable: Book.find(2) # => timestamps of Song(1) and Book(2) are updated
+
+    *Severin Schoepke*
+
 *   Improve formatting of migration exception messages: make them easier to read
     with line breaks before/after, and improve the error for pending migrations.
 
diff --git a/activerecord/lib/active_record/associations/builder/belongs_to.rb b/activerecord/lib/active_record/associations/builder/belongs_to.rb
index ac387d377d..e8e36e7cd0 100644
--- a/activerecord/lib/active_record/associations/builder/belongs_to.rb
+++ b/activerecord/lib/active_record/associations/builder/belongs_to.rb
@@ -91,7 +91,13 @@ module ActiveRecord::Associations::Builder
       old_foreign_id = o.changed_attributes[foreign_key]
 
       if old_foreign_id
-        klass      = o.association(name).klass
+        association = o.association(name)
+        reflection = association.reflection
+        if reflection.polymorphic?
+          klass = o.public_send("#{reflection.foreign_type}_was").constantize
+        else
+          klass = association.klass
+        end
         old_record = klass.find_by(klass.primary_key => old_foreign_id)
 
         if old_record
diff --git a/activerecord/test/cases/timestamp_test.rb b/activerecord/test/cases/timestamp_test.rb
index 2953b2b2be..717e0e1866 100644
--- a/activerecord/test/cases/timestamp_test.rb
+++ b/activerecord/test/cases/timestamp_test.rb
@@ -272,36 +272,62 @@ class TimestampTest < ActiveRecord::TestCase
     assert_not_equal time, old_pet.updated_at
   end
 
-  def test_changing_parent_of_a_record_touches_both_new_and_old_polymorphic_parent_record
-    klass = Class.new(ActiveRecord::Base) do
-      def self.name; 'Toy'; end
+  def test_changing_parent_of_a_record_touches_both_new_and_old_polymorphic_parent_record_changes_within_same_class
+    car_class = Class.new(ActiveRecord::Base) do
+      def self.name; 'Car'; end
     end
 
-    wheel_klass = Class.new(ActiveRecord::Base) do
+    wheel_class = Class.new(ActiveRecord::Base) do
       def self.name; 'Wheel'; end
       belongs_to :wheelable, :polymorphic => true, :touch => true
     end
 
-    toy1 = klass.find(1)
-    toy2 = klass.find(2)
+    car1 = car_class.find(1)
+    car2 = car_class.find(2)
 
-    wheel = wheel_klass.new
-    wheel.wheelable = toy1
-    wheel.save!
+    wheel = wheel_class.create!(wheelable: car1)
 
     time = 3.days.ago.at_beginning_of_hour
 
-    toy1.update_columns(updated_at: time)
-    toy2.update_columns(updated_at: time)
+    car1.update_columns(updated_at: time)
+    car2.update_columns(updated_at: time)
 
-    wheel.wheelable = toy2
+    wheel.wheelable = car2
     wheel.save!
 
-    toy1.reload
-    toy2.reload
+    assert_not_equal time, car1.reload.updated_at
+    assert_not_equal time, car2.reload.updated_at
+  end
+
+  def test_changing_parent_of_a_record_touches_both_new_and_old_polymorphic_parent_record_changes_with_other_class
+    car_class = Class.new(ActiveRecord::Base) do
+      def self.name; 'Car'; end
+    end
+
+    toy_class = Class.new(ActiveRecord::Base) do
+      def self.name; 'Toy'; end
+    end
+
+    wheel_class = Class.new(ActiveRecord::Base) do
+      def self.name; 'Wheel'; end
+      belongs_to :wheelable, :polymorphic => true, :touch => true
+    end
+
+    car = car_class.find(1)
+    toy = toy_class.find(3)
+
+    wheel = wheel_class.create!(wheelable: car)
+
+    time = 3.days.ago.at_beginning_of_hour
+
+    car.update_columns(updated_at: time)
+    toy.update_columns(updated_at: time)
+
+    wheel.wheelable = toy
+    wheel.save!
 
-    assert_not_equal time, toy1.updated_at
-    assert_not_equal time, toy2.updated_at
+    assert_not_equal time, car.reload.updated_at
+    assert_not_equal time, toy.reload.updated_at
   end
 
   def test_clearing_association_touches_the_old_record