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.

Merge the ‘available=true’ condition to all ActiveRecord’s queries
while fetching the records.

With reference Rails extending
ActiveRecord::Basehttp://stackoverflow.com/questions/2328984/rails-extending-activerecordbase,
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)