Add a failing test for assigning nil to a polymorphic belongs_to not nullifying its _type column

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -578,6 +578,19 @@ def test_polymorphic_assignment_with_primary_key_updates_foreign_id_field_for_ne
     assert_nil essay.writer_id
   end
 
+  def test_polymorphic_assignment_with_nil
+    essay = Essay.new
+    assert_nil essay.writer_id
+    assert_nil essay.writer_type
+
+    essay.writer_id = 1
+    essay.writer_type = 'Author'
+
+    essay.writer = nil
+    assert_nil essay.writer_id
+    assert_nil essay.writer_type
+  end
+
   def test_belongs_to_proxy_should_not_respond_to_private_methods
     assert_raise(NoMethodError) { companies(:first_firm).private_method }
     assert_raise(NoMethodError) { companies(:second_client).firm.private_method }