1 files changed, 10 insertions, 7 deletions
diff --git a/activemodel/lib/active_model/attribute_mutation_tracker.rb b/activemodel/lib/active_model/attribute_mutation_tracker.rb
index c67e1b809a..6abf37bd44 100644
--- a/activemodel/lib/active_model/attribute_mutation_tracker.rb
+++ b/activemodel/lib/active_model/attribute_mutation_tracker.rb
@@ -11,6 +11,10 @@ module ActiveModel
       @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)
@@ -23,7 +27,7 @@ module ActiveModel
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
         change = change_to_attribute(attr_name)
         if change
-          result[attr_name] = change
+          result.merge!(attr_name => change)
         end
       end
     end
@@ -65,13 +69,8 @@ module ActiveModel
       forced_changes << attr_name.to_s
     end
 
-    # TODO Change this to private once we've dropped Ruby 2.2 support.
-    # Workaround for Ruby 2.2 "private attribute?" warning.
-    protected
-
-      attr_reader :attributes, :forced_changes
-
     private
+      attr_reader :attributes, :forced_changes
 
       def attr_names
         attributes.keys
@@ -81,6 +80,10 @@ module ActiveModel
   class NullMutationTracker # :nodoc:
     include Singleton
 
+    def changed_attribute_names(*)
+      []
+    end
+
     def changed_values(*)
       {}
     end