テストデータジェネレータに Fabrication-gem を使う ref: http://qiita.com/hal_sk/items/e301259c8c49f5266699

file0.txt

group :test do
  gem 'fabrication' #追加
end

user.rb

class User
  include Mongoid::Document
  validates_presence_of :name

  field :name,               :type => String
  field :email,              :type => String, :default => ""
  field :password, :type => String, :default => ""

  embeds_many :user_accounts
end

user_account.rb

class UserAccount
  include Mongoid::Document
  field :provider,            :type => String
  field :uid,                 :type => String
  field :token,               :type => String
  field :auth_response,       :type => String
  FACEBOOK = 'facebook'.freeze
  TWITTER = 'twitter'.freeze

  embeded_in :user, :class_name => "User"
end

user_account_spec.rb

require 'spec_helper'

describe UserAccount do
  describe '.all' do
    before do
      Fabricate(:user)
      #FactoryGirl.create(:user_account, user: user)
    end
    subject { User.find(1) }
    it { should have(1).user_accounts }
  end

end

user_fabricator.rb

Fabricator(:user) do
  id 1
  name 'user'
  email 'hal@test.co.jp'
  password 'password'
  user_accounts(count:1){|attr, i| Fabricate(:user_account) }
end