Merge pull request #32497 from eugeneius/mutation_tracker_changed_attribute_names

Avoid generating full changes hash on every save

activemodel/lib/active_model/attribute_mutation_tracker.rb

@@ -11,6 +11,10 @@ def initialize(attributes)
       @forced_changes = Set.new
     end
 
+    def changed_attribute_names
+      attr_names.select { |attr_name| changed?(attr_name) }
+    end
+
     def changed_values
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
         if changed?(attr_name)
@@ -76,6 +80,10 @@ def attr_names
   class NullMutationTracker # :nodoc:
     include Singleton
 
+    def changed_attribute_names(*)
+      []
+    end
+
     def changed_values(*)
       {}
     end

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -114,7 +114,7 @@ def changes_to_save
 
       # Alias for +changed+
       def changed_attribute_names_to_save
-        changes_to_save.keys
+        mutations_from_database.changed_attribute_names
       end
 
       # Alias for +changed_attributes+