demo.rb

after_initialize :init_many_to_many_counters

def init_many_to_many_counters
  setup_many_to_many_counts(:categories, :tags)
end

may_to_many_counter_cache.rb

def setup_many_to_many_counts(args = {})
    args.each do |k,v|
      v = "#{self.class.downcase.pluralize}_count" if v.nil?
      self.class_eval{
        define_method "#{k}_with_count=" do |items|
          has_many_items = self.method(k).call().dup
          self.method(k).call(items)
          has_many_update_counters(k, v, has_many_items)
        end
      }
    end
  end

  def has_many_update_counters(table,col,items)
    self_items = self.method(table).call()
    binding.pry
    unless(items == self_items)
      model = table.to_s.singularize.camelize.constantize

      decrement_arr = items - self_items
      increment_arr = self_items - items

      model.decrement_counter(col, decrement_arr.map{|d| d.id})
      model.increment_counter(col, increment_arr.map{|i| i.id})
    end
  end