Rails/Bundler precompile vs lazy compile

In the config/application.rb file in a Rails app, there's the开发者_StackOverflow中文版 following section of code:

if defined?(Bundler)
    # If you precompile assets before deploying to production, use this line
    Bundler.require *Rails.groups(:assets => %w(development test))
    # If you want your assets lazily compiled in production, use this line
    # Bundler.require(:default, :assets, Rails.env)
end

I'm perhaps not clear what Bundler.require is doing. I was under the impression that it required the specified sections in the Gemfile, but I'm not clear as to why Bundler.require *Rails.groups(...) causes it to precompile and Bundler.require(...) causes assets to be lazily loaded.

These lines don't actually change how your assets are used.

The first line,

Bundler.require *Rails.groups(:assets => %w(development test))

only loads gems from the assets group in your development and test environment. This means that things like sass-rails and uglifier won't be available in production, which then means that you won't be able to properly compile/minify/whatever your assets on the fly in production if you're making use of those gems.

On the other hand,

Bundler.require(:default, :assets, Rails.env)

will load the assets group in any environment, making those gems available in production to do asset compilation/minification/whatever on the fly.

So, as stated above, these lines don't actually change the behaviour of your asset pipeline - it simply means that you should use the first if you're going to precompile your assets for production, or use the second if you're going to lazily compile in production.