新手问题 Why Rails 4 remove assets group？
Rails 4.0 removed the assets group from Gemfile. You'd need to remove that line from your Gemfile when upgrading. You should also update your application file (in config/application.rb):
# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(:default, Rails.env)
我在 stackoverflow 上找到了答案，但是还是不太理解。
Previously the assets group existed to avoid unintended compilation-on-demand in production. As Rails 4 doesn't behave like that anymore, it made sense to remove the asset group.
Previously the assets group existed to avoid unintended compilation-on-demand in production
这一句是什么意思？
Rails 4 是强制precompile your assets, 莫非 Rails 3 不是这样吗。