add_counter_cache_methods

# File activerecord/lib/active_record/associations/builder/belongs_to.rb, line 28
    def self.add_counter_cache_methods(mixin)
      return if mixin.method_defined? :belongs_to_counter_cache_after_update

      mixin.class_eval do
        def belongs_to_counter_cache_after_update(reflection)
          foreign_key  = reflection.foreign_key
          cache_column = reflection.counter_cache_column

          if (@_after_create_counter_called ||= false)
            @_after_create_counter_called = false
          elsif attribute_changed?(foreign_key) && !new_record? && reflection.constructable?
            model           = reflection.klass
            foreign_key_was = attribute_was foreign_key
            foreign_key     = attribute foreign_key

            if foreign_key && model.respond_to?(:increment_counter)
              model.increment_counter(cache_column, foreign_key)
            end
            if foreign_key_was && model.respond_to?(:decrement_counter)
              model.decrement_counter(cache_column, foreign_key_was)
            end
          end
        end
      end
    end