conteudo.rb

# -*- encoding: utf-8 -*-
class Conteudo
  include Mongoid::Document
  field :titulo, :type => String
  field :tags, :type => Array, :default => []
end

tag.rb

# -*- encoding: utf-8 -*-
class Tag

  include Mongoid::Document

  collection_name = "tags"

  def self.gerar
    map = <<-JAVASCRIPT
      function() { 
        this.tags.forEach( function(tag) {
          emit(tag, { quantidade: 1 } );
        }) 
      }
    JAVASCRIPT
    reduce = <<-JAVASCRIPT
      function(key, values) {
         var soma = 0;
         values.forEach( function(value) {
           soma += value.quantidade;
         } );
         return { quantidade: soma }; 
      }
    JAVASCRIPT
    Conteudo.collection.map_reduce(map, reduce, {out: collection_name})
  end

  def self.instantiate(attrs)
    super(attrs['value'].merge('valor' => attrs['_id']))
  end

  def self.hereditary?
    return false
  end

  Mongoid::Persistence.instance_methods.each do |method|
    undef_method method
  end

end

tags.rake

# -*- encoding: utf-8 -*-
namespace :tags do

  desc 'gera tags'
  task :tags => :environment do
    Tag.gerar
  end

end