Fix: Inconsistent Polymorphic Association Autosave

activerecord/lib/active_record/autosave_association.rb

@@ -463,7 +463,8 @@ def save_has_one_association(reflection)
       # If the record is new or it has changed, returns true.
       def _record_changed?(reflection, record, key)
         record.new_record? ||
-          association_foreign_key_changed?(reflection, record, key) ||
+          (association_foreign_key_changed?(reflection, record, key) ||
+          inverse_polymorphic_association_changed?(reflection, record)) ||
           record.will_save_change_to_attribute?(reflection.foreign_key)
       end
 
@@ -473,6 +474,14 @@ def association_foreign_key_changed?(reflection, record, key)
         record._has_attribute?(reflection.foreign_key) && record._read_attribute(reflection.foreign_key) != key
       end
 
+      def inverse_polymorphic_association_changed?(reflection, record)
+        return false unless reflection.inverse_of&.polymorphic?
+
+        class_name = record._read_attribute(reflection.inverse_of.foreign_type)
+
+        reflection.active_record != record.class.polymorphic_class_for(class_name)
+      end
+
       # Saves the associated record if it's new or <tt>:autosave</tt> is enabled.
       #
       # In addition, it will destroy the association if it was marked for destruction.

activerecord/test/cases/autosave_association_test.rb

@@ -36,6 +36,9 @@
 require "models/reply"
 require "models/attachment"
 require "models/translation"
+require "models/chef"
+require "models/cake_designer"
+require "models/drink_designer"
 
 class TestAutosaveAssociationsInGeneral < ActiveRecord::TestCase
   def test_autosave_works_even_when_other_callbacks_update_the_parent_model
@@ -1276,6 +1279,8 @@ def destroy(*args)
 end
 
 class TestAutosaveAssociationOnAHasOneAssociation < ActiveRecord::TestCase
+  fixtures :chefs, :cake_designers, :drink_designers
+
   self.use_transactional_tests = false unless supports_savepoints?
 
   def setup
@@ -1416,6 +1421,34 @@ def test_mark_for_destruction_is_ignored_without_autosave_true
 
     assert_not_predicate ship, :valid?
   end
+
+  def test_recognises_inverse_polymorphic_association_changes_with_same_foreign_key
+    chef_a = chefs(:gordon_ramsay)
+    chef_b = chefs(:marco_pierre_white)
+
+    cake_designer_a = cake_designers(:flora) # id: 1
+    cake_designer_a.update!(chef: chef_a)
+    cake_designer_b = cake_designers(:frosty) # id: 3
+    cake_designer_b.update!(chef: chef_b)
+
+    drink_designer_a = drink_designers(:turner) # id: 1
+    drink_designer_b = drink_designers(:sparrow) # id: 2
+
+    swap_chefs(cake_designer_b, drink_designer_b)
+    assert_predicate cake_designer_b.reload.chef, :present?
+    assert_not_predicate drink_designer_b.reload.chef, :present?
+
+    swap_chefs(cake_designer_a, drink_designer_a)
+    assert_predicate cake_designer_a.reload.chef, :present?
+    assert_not_predicate drink_designer_a.reload.chef, :present?
+  end
+
+  private
+    def swap_chefs(cake_designer, drink_designer)
+      drink_designer.chef = cake_designer.chef
+      drink_designer.save!
+      cake_designer.save!
+    end
 end
 
 class TestAutosaveAssociationOnAHasOneThroughAssociation < ActiveRecord::TestCase

activerecord/test/fixtures/cake_designers.yml

@@ -0,0 +1,5 @@
+flora:
+  id: 1
+
+frosty:
+  id: 2

activerecord/test/fixtures/chefs.yml

@@ -0,0 +1,5 @@
+gordon_ramsay:
+  id: 1
+
+marco_pierre_white:
+  id: 2

activerecord/test/fixtures/drink_designers.yml

@@ -0,0 +1,5 @@
+turner:
+  id: 1
+
+sparrow:
+  id: 3