DirtyModel uses a hash to keep track of any changes made to attributes

of an instance. When using the attribute_will_change! method, you must
supply a string and not a symbol or the *_changed? method will break
(because it is looking for the attribute name as a string in the keys
of the underlying hash). To remedy this, I simply made the underlying
hash a HashWithIndifferentAccess so it won't matter if you supply
the attribute name as a symbol or string to attribute_will_change!.

activemodel/CHANGELOG.md

@@ -1,5 +1,10 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Updated the DirtyModel *_changed? method to be indifferent between using 
+    symbols and strings as keys.
+
+    *William Myers*
+
 *   Add `ActiveModel::Validations::AbsenceValidator`, a validator to check the
     absence of attributes.
 

activemodel/lib/active_model/dirty.rb

@@ -139,7 +139,7 @@ def previous_changes
     #   person.name = 'robert'
     #   person.changed_attributes # => {"name" => "bob"}
     def changed_attributes
-      @changed_attributes ||= {}
+      @changed_attributes ||= ActiveSupport::HashWithIndifferentAccess.new
     end
 
     private

activemodel/test/cases/dirty_test.rb

@@ -3,11 +3,12 @@
 class DirtyTest < ActiveModel::TestCase
   class DirtyModel
     include ActiveModel::Dirty
-    define_attribute_methods :name, :color
+    define_attribute_methods :name, :color, :size
 
     def initialize
       @name = nil
       @color = nil
+      @size = nil
     end
 
     def name
@@ -28,6 +29,15 @@ def color=(val)
       @color = val
     end
 
+    def size
+      @size
+    end
+
+    def size=(val)
+      attribute_will_change!(:size) unless val == @size
+      @size = val
+    end
+
     def save
       @previously_changed = changes
       @changed_attributes.clear
@@ -114,4 +124,9 @@ def save
     assert_equal ["Otto", "Mr. Manfredgensonton"], @model.name_change
     assert_equal @model.name_was, "Otto"
   end
+
+  test "using attribute_will_change! with a symbol" do
+    @model.size = 1
+    assert @model.size_changed?
+  end
 end