#app/models/image.rb
class Image
  include Mongoid::Document
  include Mongoid::Timestamps

  field :message,:type => String
  field :tags,:type => Array

  def self.images_for tag
    Image.any_in(:tags => [tag])
  end
end

#routes.rb
match "tag/:tag" => "images#tag"

#/app/controller/images_controller.rb
class ImagesController < ApplicationController
  # GET /tag
  # GET /tag.xml
  def tag
    @images = Image.images_for params[:tag]

    respond_to do |format|
      format.html { render "index" }
      format.xml  { render :xml => @images }
    end
  end
end

这有效,但我仍然对Image.any_in map / reduce的性能有点怀疑.我认为可能有一个更好的解决方案,但尚未找到它.