Always reset changed attributes in becomes

When ```becomes``` changes @attributes it should also change
@changed_attributes. Otherwise we'll experience a kind of split head situation
where attributes are coming from ```self```, but changed_attributes is coming
from ```klass.new```. This affects the inheritance_colmn as it's changed by new
for example.

Fixes #16881

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/test/cases/inheritance_test.rb b/activerecord/test/cases/inheritance_test.rb
index 6b18bfe84f..3268555cb8 100644
--- a/activerecord/test/cases/inheritance_test.rb
+++ b/activerecord/test/cases/inheritance_test.rb
@@ -121,6 +121,12 @@ class InheritanceTest < ActiveRecord::TestCase
     assert_kind_of Cabbage, cabbage
   end
 
+  def test_becomes_and_change_tracking_for_inheritance_columns
+    cucumber = Vegetable.find(1)
+    cabbage = cucumber.becomes!(Cabbage)
+    assert_equal ['Cucumber', 'Cabbage'], cabbage.custom_type_change
+  end
+
   def test_alt_becomes_bang_resets_inheritance_type_column
     vegetable = Vegetable.create!(name: "Red Pepper")
     assert_nil vegetable.custom_type