spec_helper.rb

# This file is copied to spec/ when you run 'rails generate rspec:install'
ENV["RAILS_ENV"] ||= 'test'
require File.expand_path("../../config/environment", __FILE__)
require 'rspec/rails'

# Requires supporting ruby files with custom matchers and macros, etc,
# in spec/support/ and its subdirectories.
Dir[Rails.root.join("spec/support/**/*.rb")].each {|f| require f}
require 'shoulda'

RSpec.configure do |config|
  config.mock_with :rspec
end

user.rb

class User
  include Mongoid::Document
  # Include default devise modules. Others available are:
  # :token_authenticatable, :confirmable, :lockable and :timeoutable, :trackable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :validatable
  attr_protected :_id
end

user_spec.rb

require 'spec_helper'

describe User do
  it {should_not allow_mass_assignment_of(:_id)}
end