1 files changed, 12 insertions, 5 deletions
diff --git a/activemodel/lib/active_model/attribute_mutation_tracker.rb b/activemodel/lib/active_model/attribute_mutation_tracker.rb
index 8e92c8807f..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
@@ -35,10 +39,6 @@ module ActiveModel
       end
     end
 
-    def changed_attribute_names
-      attr_names.select { |attr| changed?(attr) }
-    end
-
     def any_changes?
       attr_names.any? { |attr| changed?(attr) }
     end
@@ -80,6 +80,10 @@ module ActiveModel
   class NullMutationTracker # :nodoc:
     include Singleton
 
+    def changed_attribute_names(*)
+      []
+    end
+
     def changed_values(*)
       {}
     end
@@ -108,5 +112,8 @@ module ActiveModel
 
     def original_value(*)
     end
+
+    def force_change(*)
+    end
   end
 end