1 files changed, 5 insertions, 2 deletions
diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index d327913824..3f2fd12db7 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -30,7 +30,7 @@ module ActiveModel
   #
   #     include ActiveModel::Dirty
   #
-  #     define_attribute_methods [:name]
+  #     define_attribute_methods :name
   #
   #     def name
   #       @name
@@ -83,6 +83,7 @@ module ActiveModel
   # in-place attributes.
   #
   #   person.name_will_change!
+  #   person.name_change    # => ['Bill', 'Bill']
   #   person.name << 'y'
   #   person.name_change    # => ['Bill', 'Billy']
   module Dirty
@@ -151,13 +152,15 @@ module ActiveModel
 
       # Handle <tt>*_will_change!</tt> for +method_missing+.
       def attribute_will_change!(attr)
+        return if attribute_changed?(attr)
+
         begin
           value = __send__(attr)
           value = value.duplicable? ? value.clone : value
         rescue TypeError, NoMethodError
         end
 
-        changed_attributes[attr] = value unless changed_attributes.include?(attr)
+        changed_attributes[attr] = value
       end
 
       # Handle <tt>reset_*!</tt> for +method_missing+.