如何获取具有has_and_belongs_to_many关联的FactoryGirl模型属性?

我在努力 FactoryGirl.attributes_for(:trip) 与HABTM协会 :countries 因为控制器测试失败- :国家/地区 在中缺席 :trip 属性):

跳闸控制器:

class TripsController < ApplicationController
  def create
    trip = Trip.new(create_params)
    if trip.save
      redirect_to trips_path, notice: 'Trip successfully created.'
    else
      redirect_to :back, alert: trip.errors.full_messages.join('<br>').html_safe
    end
  end

  def create_params
    params.require(:trip).permit(:end_date, :description, country_ids: [], countries: [])
  end
end

RSpec跳闸控制器测试:

describe TripsController do
  describe 'POST #create' do
    before { post :create, trip: attributes_for(:trip) }
    it { is_expected.to redirect_to trips_path }
  end
end

跳闸模式:

class Trip < ActiveRecord::Base
  # Associations
  has_and_belongs_to_many :countries

  #Validations
  validate :require_at_least_one_country

  private

  def require_at_least_one_country
    if country_ids.empty? && countries.count == 0
      errors.add(:base, 'Please select at least one country')
    end
  end
end

跳闸工厂:

FactoryGirl.define do
  factory :trip do
    description  { Faker::Lorem.sentence }
    end_date  { DateTime.now + 1.day }

    after(:build) do |trip, evaluator|
      trip.countries << FactoryGirl.create(:country, :with_currencies)
    end
  end
end

宝石文件:

factory_girl_rails (4.5.0)

尝试过: http://makandracards.com/jan0sch/11111-rails-factorygirl-and-has_and_belongs_to_many ,但毫无用处。