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                # 2.5 練習 電子書中有練習的答案,如果想閱讀參考答案,請[購買電子書](http://railstutorial-china.org/#purchase)。 1. [代碼清單 2.18](#listing-validates-content-presence) 為微博內容添加了一個存在性驗證,以此確保微博不能為空。驗證這個規則確實能實現如[圖 2.19](#fig-micropost-content-cant-be-blank) 所示的效果。 2. 修改[代碼清單 2.19](#listing-toy-user-presence-valications),把 `FILL_IN` 改成合適的代碼,驗證用戶模型的 `name` 和 `email` 屬性都存在([圖 2.20](#fig-user-presence-validations))。 ##### 代碼清單 2.18:微博內容的存在性驗證 app/models/micropost.rb ``` class Micropost < ActiveRecord::Base belongs_to :user validates :content, length: { maximum: 140 }, presence: true end ``` ##### 代碼清單 2.19:在用戶模型中加入存在驗證 app/models/user.rb ``` class User < ActiveRecord::Base has_many :microposts validates FILL_IN, presence: true validates FILL_IN, presence: true end ``` ![micropost content cant be blank](https://box.kancloud.cn/2016-05-11_5732bcf248ad6.png)圖 2.19:微博模型存在性驗證的效果![user presence validations](https://box.kancloud.cn/2016-05-11_5732bcf25b4fa.png)圖 2.20:用戶模型中存在性驗證的效果
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