如何使用rspec-rails 3.0.0和shoulda-matcher 2.5.0测试条件验证

class Event < ActiveRecord::Base
  validates :name,:location,:time,:city,:zipcode,:user,:state,presence: true
  validates :post_text,presence: true,if: :happened?
  validates :pre_text,unless: :happened?

  belongs_to :community
  belongs_to :user
  belongs_to :state

  def happened?
    (self.time < Time.now) ? true : false
  end
end

我的event_spec.rb看起来像：

require 'spec_helper'

describe Event do
  before { @event = FactoryGirl.build(:future_event) }

  subject { @event }

  it { should validate_presence_of(:time) }
  it { should validate_presence_of(:name) }
  it { should validate_presence_of(:location) }
  it { should validate_presence_of(:user) }
  it { should validate_presence_of(:city) }
  it { should validate_presence_of(:state) }
  it { should validate_presence_of(:zipcode) }

  it { should belong_to(:state) }
  it { should belong_to(:user) }

  it 'has pre_text if future event' do
     expect(FactoryGirl.create(:future_event)).to be_valid
  end

  it 'has post_text if past event' do
    expect(FactoryGirl.create(:past_event)).to be_valid
  end
end

但是当我运行测试时,我的{should validate_presence_of(：time)}块失败,因为它继续运行剩余的验证并在条件验证上抛出异常,因为self.time为nil.我通过检查time.present实现了一个hacky修复程序？在发生了什么？方法.但是,任何人都可以帮助我理解测试需要存在另一个字段的条件验证的最佳方法是什么？