article.rb

class Article
  include Mongoid::Document
  include Mongoid::Timestamps

  include Mongoid::Document::Taggable # https://github.com/chebyte/mongoid-simple-tags

  ## Fields ##
  field :title,           type: String

  ## Accessors ##
  attr_accessor :body

  ## Associations ##
  belongs_to :user, index: true, inverse_of: :actu
  
  ## Validations##
  validates :title, presence: true
  validates :user, presence: true

  ## Index ##
  index({ title: 1 }, { unique: true, background: true })

  ## Scopes ##

  ## Tire ##
  include Tire::Model::Search
  include Tire::Model::Callbacks

  index_name "#{Tire::Model::Search.index_prefix}actus" if Rails.env == "test"
  tire do
    mapping do
      indexes :title, boost: 100
      indexes :tag_list
    end
  end

  def to_indexed_json
    {
      title: title,
      tag_list: tag_list
    }.to_json
  end

end

article_spec.rb

require 'spec_helper'

describe Article do 
 describe ".tags", :focus do
    let!(:index_name) {"#{Tire::Model::Search.index_prefix}articles"}
    let!(:index) { Tire::Index.new(index_name) }

    before(:each) do
      index.delete
      create(:actu, tags: ["Sony", "Gaikai"])
      create(:actu, tag_list: "Nintendo, Gamescom")
      index.refresh
    end

    it "Should return all tags" do
      search = Tire.search([index_name]) do
        facet 'global-tags' do
          terms :tag_list
        end
      end

      search.results.facets['global-tags']['terms'].should eq([{"term"=>"sony", "count"=>1}, {"term"=>"nintendo", "count"=>1}, {"term"=>"gamescom", "count"=>1}, {"term"=>"gaikai", "count"=>1}])
    end

    it "Should return related article" do
      search = Tire.search([index_name], load: true) do
        filter :terms, tag_list: ['nintendo']
      end

      search.results.first.tags.should be_an_instance_of(Array)
      search.results.first.tags.should include('Nintendo')
      search.results.first.tags.should_not include('Sony')
    end
  end
end