update_counter_in_memory

# File activerecord/lib/active_record/associations/has_many_association.rb, line 114
        def update_counter_in_memory(difference, reflection = reflection())
          if counter_must_be_updated_by_has_many?(reflection)
            counter = cached_counter_attribute_name(reflection)
            owner[counter] += difference
            owner.send(:clear_attribute_changes, counter) # eww
          end
        end