Example for testing mongoid assocications with Rspec/shoulda

album_spec.rb

require File.expand_path(File.dirname(__FILE__) + '/../spec_helper')

describe Album do
  before :each do
    @website = Factory(:website)
    @album = @website.albums.build(Factory(:album).attributes)
    @album.save
  end
  it "should be valid" do
    @album.should be_valid
  end
  
  describe "validations" do
    it { should validate_presence_of(:title) }
    it { should validate_presence_of(:body) }
    it { should validate_presence_of(:body_html) }
  end
  describe "associations" do
    it "should be embedded in a website" do
      association = Page.associations['website']
      association.klass.should ==(Website)
      association.association.should ==(Mongoid::Associations::EmbeddedIn)
      association.inverse_of.should ==(:pages)
    end
  end
end

spec_helper.rb

# This file is copied to ~/spec when you run 'ruby script/generate rspec'
# from the project root directory.
ENV["RAILS_ENV"] ||= 'test'
require 'factory_girl'
require File.dirname(__FILE__) + "/../config/environment" unless defined?(Rails.root)
require 'rails/test_help'
require 'rspec/rails'
require 'shoulda'
require 'shoulda/active_model'
require 'mongoid'
require 'webrat'
require 'rspec/expectations'

# Requires supporting files with custom matchers and macros, etc,
# in ./support/ and its subdirectories.
Dir["#{File.dirname(__FILE__)}/support/**/*.rb"].each {|f| require f}

Rspec.configure do |config|
  config.include Rspec::Matchers
  # requires this branch:
  # http://github.com/TomK32/shoulda/commits/master
  config.include Shoulda::ActiveModel::Matchers
  
  config.mock_with :rspec

end

website_spec.rb

require File.expand_path(File.dirname(__FILE__) + '/../spec_helper')

describe Website do
  before(:each) do
    @website = Factory(:website)
  end
  it "@website should be valid" do
    @website.should be_valid
  end
  describe "associations" do
    # I'm just waiting for an has_many_related :in => :user_ids like mongo_mapper has
    it "should have user_ids" do
      @website.user_ids.should be_a(Array)
      @website.user_ids.first.should be_a(String)
      User.find(@website.user_ids[0]).id.should ==(@website.user_ids[0])
    end
    it "should embed a theme" do
      association = Website.associations['theme']
      association.klass.should ==(Theme)
      association.association.should ==(Mongoid::Associations::EmbedsOne)
    end
    it "should embed many albums" do
      association = Website.associations['albums']
      association.klass.should ==(Album)
      association.association.should ==(Mongoid::Associations::EmbedsMany)
    end
  end
end

Oh gosh, just use the mongoid rspec gem, that does all the tricks you need.