Define to_ary on ActiveRecord::Base to return nil. Improve performance of Array#flatten under 1.9, see comment for details.

activerecord/lib/active_record/base.rb

@@ -1900,6 +1900,18 @@ def clone_attribute_value(reader_method, attribute_name)
 
     private
 
+      # Under Ruby 1.9, Array#flatten will call #to_ary (recursively) on each of the elements
+      # of the array, and then rescues from the possible NoMethodError. If those elements are
+      # ActiveRecord::Base's, then this triggers the various method_missing's that we have,
+      # which significantly impacts upon performance.
+      #
+      # So we can avoid the method_missing hit by explicitly defining #to_ary as nil here.
+      #
+      # See also http://tenderlovemaking.com/2011/06/28/til-its-ok-to-return-nil-from-to_ary/
+      def to_ary # :nodoc:
+        nil
+      end
+
       def set_serialized_attributes
         sattrs = self.class.serialized_attributes