overriding the ActiveModel::Dirty#attribute_change method, instead of…

… extracting the code from it

lib/mongoid/dirty.rb

@@ -20,7 +20,7 @@ module Dirty #:nodoc:
     def changes
       {}.tap do |hash|
         changed.each do |name|
-          change = [changed_attributes[name], attributes[name]] if attribute_changed?(name)
+          change = attribute_change(name)
           hash[name] = change if change[0] != change[1]
         end
       end
@@ -72,5 +72,23 @@ def setters
         end
       end
     end
+
+    private
+
+    # Get the current value for the specified attribute, if the attribute has changed.
+    #
+    # @note This is overriding the AM::Dirty implementation to read from the mongoid 
+    #   attributes hash, which may contain a serialized version of the attributes data. It is
+    #   necessary to read the serialized version as the changed value, to allow updates to
+    #   the MongoDB document to persist correctly. For example, if a DateTime field is updated
+    #   it must be persisted as a UTC Time. 
+    #
+    # @return [ Object ] The current value of the field, or nil if no change made.
+    #
+    # @since 2.1.0
+    def attribute_change(attr)
+      [changed_attributes[attr], attributes[attr]] if attribute_changed?(attr)
+    end
+
   end
 end