warning: toplevel constant Model referenced by NS::Model

今天遇到了這個問題，在網上搜一下找到一個滿詳盡的解釋，茅塞頓開：

Your User::File class is not loaded. You have to require it (e.g. in user.rb).

The following happens when ruby/rails sees User::Info and evaluates it (simplified; only User is defined yet).

• check if User::Info is defined - it is not (yet)
• check if Info is defined - it is not (yet)
• uninitialized constant -> do rails magic to find the user/info.rb file and require it
• return User::Info

Now lets do it again for User::File

• check if User::File is defined - it is not (yet)
• check if File is defined - it is (because ruby has a built in File class)!
• produce a warning, because we've been asked for User::File but got ::File
• return ::File

裡面提到了要手動 Require NameSpace 下的 Model，但是如果 NS 下的 model 本身有繼承到 NS 外的 Model，要確保在父類別宣告後建立才行。所以最好是在父類別檔案的結尾處 require。