我发现这个问题是由DatabaseCleaner引起的，解决方法是注解掉Databasecleaner的一些设置。

RSpec.configure do |config|
  config.before(:suite) do
    #DatabaseCleaner[:mongoid].clean_with(:deletion)
  end
  config.before(:each) do
    # DatabaseCleaner[:mongoid].strategy = :deletion
  end
  config.before(:each, :js => true) do
    #DatabaseCleaner[:mongoid].strategy = :deletion
  end
  config.before(:each) do
    DatabaseCleaner.start
  end
  config.after(:each) do
    # DatabaseCleaner.clean
  end
  config.before(:all) do
    # DatabaseCleaner.start
  end
  config.after(:all) do
    # DatabaseCleaner.clean
  end
end