update_counter

# File activerecord/lib/active_record/associations/has_many_association.rb, line 82
        def update_counter(difference, reflection = reflection())
          if has_cached_counter?(reflection)
            counter = cached_counter_attribute_name(reflection)
            owner.class.update_counters(owner.id, counter => difference)
            owner[counter] += difference
            owner.changed_attributes.delete(counter) # eww
          end
        end