Bugfix: Ensure has_one associations saved when part of CPK has changed

If a has_one association uses a composite primary key, and part of the
composite primary key changes on the owner, these changes need to be
reflected on the belonging object's foreign key.

This was not working previously, because `#_record_changed?` was not
equipped to handle composite primary key associations, so we were not
recognizing that the belonging object's foreign key needed to be updated
when the owner's primary key changed.

activerecord/lib/active_record/autosave_association.rb

@@ -482,7 +482,10 @@ def _record_changed?(reflection, record, key)
       def association_foreign_key_changed?(reflection, record, key)
         return false if reflection.through_reflection?
 
-        record._has_attribute?(reflection.foreign_key) && record._read_attribute(reflection.foreign_key) != key
+        foreign_key = Array(reflection.foreign_key)
+        return false unless foreign_key.all? { |key| record._has_attribute?(key) }
+
+        foreign_key.map { |key| record._read_attribute(key) } != Array(key)
       end
 
       def inverse_polymorphic_association_changed?(reflection, record)

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -376,6 +376,16 @@ def test_belongs_to_with_inverse_association_for_composite_primary_key
     assert_equal order_id, book.order_id
   end
 
+  def test_should_set_composite_foreign_key_on_association_when_key_changes_on_associated_record
+    book = Cpk::Book.create!(id: [1, 2], title: "The Well-Grounded Rubyist")
+    order = Cpk::Order.create!(id: [1, 2], book: book)
+
+    order.shop_id = 3
+    order.save!
+
+    assert_equal 3, book.shop_id
+  end
+
   def test_building_the_belonging_object_with_implicit_sti_base_class
     account = Account.new
     company = account.build_firm