add_counter_cache_callbacks

# File activerecord/lib/active_record/associations/builder/belongs_to.rb, line 23
      def add_counter_cache_callbacks(reflection)
        cache_column = reflection.counter_cache_column
        name         = self.name

        method_name = "belongs_to_counter_cache_after_create_for_#{name}"
        mixin.redefine_method(method_name) do
          record = send(name)
          record.class.increment_counter(cache_column, record.id) unless record.nil?
        end
        model.after_create(method_name)

        method_name = "belongs_to_counter_cache_before_destroy_for_#{name}"
        mixin.redefine_method(method_name) do
          record = send(name)
          record.class.decrement_counter(cache_column, record.id) unless record.nil?
        end
        model.before_destroy(method_name)

        model.send(:module_eval,
          "#{reflection.class_name}.send(:attr_readonly,\"#{cache_column}\".intern) if defined?(#{reflection.class_name}) && #{reflection.class_name}.respond_to?(:attr_readonly)", __FILE__, __LINE__
        )
      end