how do you test something raises an error while also testing activerecord object classes?

event_test.rb

require 'test_helper'

class EventTest < ActiveSupport::TestCase

  test "event requires a username and a timestamp" do
    e1 = Event.new
    e1.id = 1
    e1.username = nil
    e1.timestamp = Time.now
    e1.description = "some description"

    lambda { e1.save }.should_throw Error
  end

end