class Item
  include Mongoid::Document

  embeds_many :urls
  field :title
end

class Url
  include Mongoid::Document

  embedded_in :item
  field :value
  validates :value, uniqueness: true
end

describe Item do
  it 'does not save title' do
    item = Item.create
    item.urls.create(value: 'url')
    item.urls.create(value: 'url')
    expect(item.update_attributes(title: 'title')).to eq false
  end
end