如何使用多个关联构建活动记录？

Question

我在Rails3中定义了这3个模型。

class User < ActiveRecord::Base 
    has_many :questions 
    has_many :answers 

class Question < ActiveRecord::Base 
    belongs_to :user 
    has_many :answers 

class Answer < ActiveRecord::Base 
    belongs_to :user 
    belongs_to :question 

我写RSpec的是这样的：

describe "user associations" do 
    before :each do 
    @answer = @user.answers.build question: @question 
    end 

    it "should have the right associated user" do 
    @answer.user.should_not be_nil 
    end 

    it "should have the right associated question" do 
    @question.should_not be_nil 
    @answer.question.should_not be_nil #FAIL!! 
    end 

但我一直得到以下错误：

Failures: 

    1) Answer user associations should have the right associated question 
    Failure/Error: @answer.question.should_not be_nil 
     expected: not nil 
      got: nil 

我想这条线是错误的：

@answer = @user.answers.build question: @question 

但我应该如何构建答案对象？

更新：谢谢大家，我发现我应该写这样的：

require 'spec_helper' 

describe Answer do 
    before :each do 
    @user = Factory :user 
    asker = Factory :user, :user_name => 'someone' 
    @question = Factory :question, :user => asker 
    end 

    describe "user associations" do 
    before :each do 
     @answer = Factory :answer, :user => @user, :question => @question 
    end 

    it "should have the right associated user" do 
     @answer.user.should_not be_nil 
    end 

    it "should have the right associated question" do 
     @answer.question.should_not be_nil 
    end 
    end 
end 

下面是投机/ factories.rb：

Factory.define :user do |user| 
    user.user_name "junichiito" 
end 

Factory.define :question do |question| 
    question.title "my question" 
    question.content "How old are you?" 
    question.association :user 
end 

Factory.define :answer do |answer| 
    answer.content "I am thirteen." 
    answer.association :user 
    answer.association :question 
end 

Answer 1

有一次，我明确地保存@user实例，该规范不再失败。这是我的版本：

require 'spec_helper' 

describe Answer do 
    before :each do 
    @user = User.new 
    @user.save! 
    @question = @user.questions.build 
    @question.save! 

    @answer = @user.answers.build question: @question 
    @question.answers << @answer 
    end 

    it "should have the right associated user" do 
    @answer.user.should_not be_nil 
    end 

    it "should have the right associated question" do 
    @question.should_not be_nil 
    @answer.question.should_not be_nil # SUCCESS! 
    end 
end