`becomes` should clear the mutation tracker which is created in `after_initialize`

`becomes` creates new object and copies attributes from the receiver. If
new object has mutation tracker which is created in `after_initialize`,
it should be cleared since it is for discarded attributes.

But if the receiver doesn't have mutation tracker yet, it will not be
cleared properly.

It should be cleared regardless of whether the receiver has mutation
tracker or not.

Fixes #32867.

1 files changed, 11 insertions, 0 deletions

diff --git a/activerecord/test/cases/persistence_test.rb b/activerecord/test/cases/persistence_test.rb
index e0c5725944..47e9683b44 100644
--- a/activerecord/test/cases/persistence_test.rb
+++ b/activerecord/test/cases/persistence_test.rb
@@ -290,6 +290,17 @@ class PersistenceTest < ActiveRecord::TestCase
     assert_equal "The First Topic", Topic.find(copy.id).title
   end
 
+  def test_becomes_wont_break_mutation_tracking
+    topic = topics(:first)
+    reply = topic.becomes(Reply)
+
+    assert_equal 1, topic.id_in_database
+    assert_empty topic.attributes_in_database
+
+    assert_equal 1, reply.id_in_database
+    assert_empty reply.attributes_in_database
+  end
+
   def test_becomes_includes_changed_attributes
     company = Company.new(name: "37signals")
     client = company.becomes(Client)