`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.

activerecord/CHANGELOG.md

+*   `becomes` should clear the mutation tracker which is created in `after_initialize`.
+
+    Fixes #32867.
+
+    *Ryuta Kamizono*
+
 *   Allow a belonging to parent object to be created from a new record.
 
     *Jolyon Pawlyn*

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -119,7 +119,7 @@ def changed_attribute_names_to_save
 
       # Alias for +changed_attributes+
       def attributes_in_database
-        changes_to_save.transform_values(&:first)
+        mutations_from_database.changed_values
       end
 
       private

activerecord/lib/active_record/persistence.rb

@@ -373,7 +373,7 @@ def becomes(klass)
       became = klass.allocate
       became.send(:initialize)
       became.instance_variable_set("@attributes", @attributes)
-      became.instance_variable_set("@mutations_from_database", @mutations_from_database) if defined?(@mutations_from_database)
+      became.instance_variable_set("@mutations_from_database", @mutations_from_database ||= nil)
       became.instance_variable_set("@changed_attributes", attributes_changed_by_setter)
       became.instance_variable_set("@new_record", new_record?)
       became.instance_variable_set("@destroyed", destroyed?)

activerecord/test/cases/persistence_test.rb

@@ -290,6 +290,17 @@ def test_duped_becomes_persists_changes_from_the_original
     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)

activerecord/test/models/topic.rb

@@ -97,7 +97,7 @@ def destroy_children
     end
 
     def set_email_address
-      unless persisted?
+      unless persisted? || will_save_change_to_author_email_address?
         self.author_email_address = "test@test.com"
       end
     end