Simple counter cache setup for MongoMapper that supports single collection inheritance.

gistfile1.rb

module CounterCache
  extend ActiveSupport::Concern

  module ClassMethods
    def counter_cache(field)
      class_eval <<-EOF
        after_create "increment_counter_for_#{field}"
        after_destroy "decrement_counter_for_#{field}"
      EOF
    end
  end

  module InstanceMethods
    def method_missing(method, *args)
      if matches = method.to_s.match(/^(in|de)crement_counter_for_(.*)$/) then
        dir = matches[1] == "in" ? 1 : -1
        parent_association = matches[2]

        if parent = self.send(parent_association) then
          name = "#{self.class.collection_name}_count"
          if parent.respond_to?(name)
            parent.increment(name => dir)
          end
        end
      else
        super
      end
    end
  end
end