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Quick Reference:

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$ RAILS_ENV=production rake db:migrate
$ rake secret //產生secret key
$ export SECRET_KEY_BASE=[貼入上個指令產生出來的密鑰]
$ vim config/initializers/assets.rb  //加入用到的assets路徑
確認各處引用assets的方法正確
$ RAILS_ENV=production rake assets:precompile
$ vim config/environments/production.rb  //將這行改成true: config.serve_static_assets = true
$ rails s -e production

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What Is Production Mode? Why Use It?

Normally, when you run $ rails s in your project, you're starting it in development mode.

In development mode, most changes show up on the page right away, which is convenient for development.

But when the system actually goes live, we don't use development mode — we use production mode.

In production mode, rails pre-compiles the assets together and puts them under the public/ folder for access, which improves site performance.

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Starting a Rails Project in Production Mode

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0. Migrate Your Database

Production mode and development mode use separate databases, so you need to migrate again:

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$ RAILS_ENV=production rake db:migrate

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1. Edit config/secret.yml to Set the secret_key_base for Production Mode

To run a rails app properly, you need to set something called the secret_key_base, a secret key rails uses to verify browser cookies. rails has already set it up for development mode and test mode, but not for production mode — you have to do it yourself. The setting lives in the config/secret.yml file. Let's take a look at it:

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// development mode的設定
// 它使用 b1f4eb...3a9 這串密鑰（隨機生成的）
development:
  secret_key_base: b1f4eb6c1d97f627fbc0a3919fc27ab5f831200cdd0ea72317f404a2b9d878192bca27d92380f56051c60d0a364c86fe53d9b9866d487a17cf06582a783723a9
 
// test mode使用 fe9d...c3b 這串密鑰
test:
  secret_key_base: fe9d49b7edc8e3a4dfa47c0682974aca4572388e929ea3e4f037b3e8841747fc3029e030bbd102a62d372596b4d753dd0c419b7425a8dcceca3817cf7d5dbc3b
 
// production mode從執行環境裡抓 $SECRET_KEY_BASE 這個環境變數當作密鑰，但正常情況下該變數並不存在，所以出錯
production:
  secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>

You have two ways to set the secret_key_base for production mode.

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(1) The simpler way is to hardcode a fixed key:

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// 前略...
 
production:
// 刪掉原本的 <%= ... %> ，替換成隨便一串長度30以上的英數組合字串給他當密鑰
	secret_key_base: iameasonchangiamhandsomeiamthekingoftheworldnoonecandefeatmeiwantagirlfriendiamaloserqq

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(2) The safer way is to set the key in your environment:

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$ rake secret // 生成一串密鑰，顯示在終端機上
$ export SECRET_KEY_BASE=[此處貼上你剛剛產生的密鑰] // 把密鑰設定至環境變數 $SECRET_KEY_BASE

But if you stop there, the environment variable disappears when you restart your terminal, so you can write the setting into your shell config file.

For me, using zsh, the config file is ./.zshrc For Mac's default terminal shell bash, it's ./.bash_profile

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// 在.zshrc中插入下面這行
export SECRET_KEY_BASE=[此處貼上你剛剛產生的密鑰]

At this point you should be able to start your rails project in production mode without errors. Run $ rails s -e production to start the server, then browse to localhost:3000 to check the current result.

Your screen probably looks a lot like mine. Why is it so ugly? Where's the beautiful page I designed without sleeping for days?

That's because you haven't told rails which design assets you're using yet. The next steps show you how:

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2. Tell rails Which Assets Need to Be Pre-compiled

As mentioned earlier, the reason to use production mode is to improve page load performance, and the way it does that is by pre-compiling all kinds of assets. You need to manually tell rails which assets need pre-compiling. Apart from image files, every asset you use needs to be added, such as js and css files.

To do this, update the config/initializers/assets.rb file and add the paths of the assets you use:

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// 前略...
// 後面加入這一行，裡頭放入你要precompile的檔案
Rails.application.config.assets.precompile += [
  'bootstrap.js', // 請幫我compile vendor/assets/javascripts/bootstrap.js 這支檔案
	'courses/content.js', // 還有 app/assets/javascripts/courses/content.js
	'courses/chart.js',  // app/assets/javascripts/courses/chart.js 也要
	'new-index.css', // 還有這個css，它在app/assets/stylesheets/new-index.css.scss
	'login.css', // app/assets/stylesheets/login.css.erb也要
	'newcomer/*.css', // app/assets/stylesheets/newcomer/裡面的css檔都要！
	'NotoSans-Regular-ttf' // 這個是字形檔唷！它在app/assets/fonts/NotoSans-Regular-ttf
]

rails looks for your asset files in the app/assets/ and vendor/assets/ folders, and it looks in the corresponding path based on the file extension:

• js files are looked up under javascripts/
• css files are looked up under stylesheets/
• font files are looked up under fonts/
• image files are looked up under images/

But rails compiles all image files for you by default, so you don't need to add .png, .jpg, and other image files.

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3. Make Sure You're Referencing Assets Correctly

After compiling, asset filenames change, so the traditional way of referencing assets stops working. You have to use the methods rails provides instead:

• To include a js file, use <%= javascript_include_tag 'your-js-path' %>
• To include a css file, use <%= stylesheet_link_tag 'your-css-path' %>
• To include an image, use <%= image_tag 'your-image-path' %>
• To reference an asset inside a js file, use <%= asset_path('your-img-path') %>

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4. Pre-compile it!

Run this command in your project directory:

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$ RAILS_ENV=production rake assets:precompile

This pre-compiles the assets into /public/assets.

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5. Update the Production Mode Settings

The assets are compiled, but by default production mode won't let users access the files in /public/assets.

You have to edit the config/environments/production.rb file.

Around line 24, change

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config.serve_static_assets = false

to true:

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config.serve_static_assets = true

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6. Start It in Production Mode!!

Run this command in your project directory:

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$ rails s -e production

And your rails project starts in production mode!

Browse to localhost:3000 and you'll see your beautiful page, now loading much faster!

Test environment:

• OS: Mac OS X
• Rails version: 4.2.6
• Ruby version: 2.0.0p481
• Shell: zsh