Rails3-default_scope throws exception: undefined method abstract_class? for Object:Class

I would like to apply logical delete in my application(Instead of permanently deleting a record just have been marked as deleted). I have Added available column to all tables with default value true.
Now I want common place to write the following code for all models.

Write the instance method which make ‘available’ column value false when user clicks on destroy link.

1. ```
Merge the 'available=true' condition to all ActiveRecord's queries while fetching the records.

With reference Rails extending ActiveRecord::Base,
I decided to use monkey patch to do above. Created monkey patch file in config/initializer/active_record_patch.rb:

class ActiveRecord::Base
   def inactive
      update_attribute(:available, false)
   end

   default_scope :available => true
end

Getting following error when add the default_scope

/gems/ruby-1.9.2-p290/gems/activerecord-3.0.9/lib/active_record/base.rb:1212:in class_of_active_record_descendant': undefined methodabstract_class?’ for Object:Class (NoMethodError)