如何测试mongoid示波器?

我定义了以下具有作用域的User类:

class User
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::Search

  # SCOPES
  scope :all_admins, where( role: :admin)
  scope :recents, order_by(created_at: :desc)
  scope :olders, order_by(created_at: :asc)

  field :role, type: Symbol
end

如果我使用以下rspec测试:

describe 'scopes' do
  let(:admin1)            { Fabricate(:admin) }
  let(:admin2)               { Fabricate(:admin) }

  describe 'recents' do
    it 'should return the admins from most recent to older' do
      User.all_admins.recents.should eq([admin1, admin2])
    end
  end
end

我收到以下失败消息:

got: #<Mongoid::Criteria
  selector: {"role"=>:admin},
  options:  {:sort=>{"created_at"=>-1}},
  class:    User,
  embedded: false>

那么我该如何测试这个范围呢?