refine answer model

app/models/answer.rb

@@ -0,0 +1,14 @@
+class Answer < ActiveRecord::Base
+  belongs_to :quiz
+
+  validates :answer, presence: true, length: { in: 1..30 }
+  validates :right_or_wrong, presence: true
+  validates :quiz_id, presence: true
+  validate :right_answer_is_only_one_by_one_quiz
+
+  def right_answer_is_only_one_by_one_quiz
+    if quiz.answers.where(right_or_wrong: true) > 1
+      error.add(:right_answer_number, "1つの問題に対して正解は1つだけです")
+    end
+  end
+end

db/migrate/20160317000000_create_answers.rb

@@ -0,0 +1,11 @@
+class CreateAnswers < ActiveRecord::Migration
+  def change
+    create_table :answers do |t|
+      t.string :answer_text, :null => false
+      t.boolean :right_or_wrong, :null => false
+      t.belongs_to :quiz, index: true, foreign_key: true
+
+      t.timestamps null: false
+    end
+  end
+end

spec/factories/answers.rb

@@ -0,0 +1,7 @@
+FactoryGirl.define do
+  factory :answer do
+    answer_text "MyString"
+    right_or_wrong false
+    quiz_id nil
+  end
+end

spec/models/answer_spec.rb

@@ -0,0 +1,5 @@
+require 'rails_helper'
+
+RSpec.describe Answer, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end