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