`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.

4 files changed, 14 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 7ff467b033..233ee29fac 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -157,7 +157,7 @@ module ActiveRecord
       # original attribute values in the database (as opposed to the in-memory
       # values about to be saved).
       def attributes_in_database
-        changes_to_save.transform_values(&:first)
+        mutations_from_database.changed_values
       end
 
       private
diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index c2393c1fc8..a0d5f1ee9f 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -373,7 +373,7 @@ module ActiveRecord
       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?)
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)
diff --git a/activerecord/test/models/topic.rb b/activerecord/test/models/topic.rb
index fa50eeb6a4..2e386d7669 100644
--- a/activerecord/test/models/topic.rb
+++ b/activerecord/test/models/topic.rb
@@ -97,7 +97,7 @@ class Topic < ActiveRecord::Base
     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