add_counter_cache_methods

# File activerecord/lib/active_record/associations/builder/belongs_to.rb, line 29
    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_replace_counter_called ||= false)
            @_after_replace_counter_called = false
          elsif association(reflection.name).target_changed?
            if reflection.polymorphic?
              model     = attribute_in_database(reflection.foreign_type).try(:constantize)
              model_was = attribute_before_last_save(reflection.foreign_type).try(:constantize)
            else
              model     = reflection.klass
              model_was = reflection.klass
            end

            foreign_key_was = attribute_before_last_save foreign_key
            foreign_key     = attribute_in_database foreign_key

            if foreign_key && model.respond_to?(:increment_counter)
              foreign_key = counter_cache_target(reflection, model, foreign_key)
              model.increment_counter(cache_column, foreign_key)
            end

            if foreign_key_was && model_was.respond_to?(:decrement_counter)
              foreign_key_was = counter_cache_target(reflection, model_was, foreign_key_was)
              model_was.decrement_counter(cache_column, foreign_key_was)
            end
          end
        end

        private
          def counter_cache_target(reflection, model, foreign_key)
            primary_key = reflection.association_primary_key(model)
            model.unscoped.where!(primary_key => foreign_key)
          end
      end
    end