how rails override the middleware method defined by rack?

Question

This question is about rails source code.

I notice middleware method definded in railties/lib/rails/commands/server.rb ( Rails )

def middleware
  middlewares = []
  middlewares << [Rails::Rack::Debugger] if options[:debugger]
  middlewares << [::Rack::ContentLength]
  Hash.new(middlewares)
end

I think it returns a blank hash {} . It overrides the method with same name which definded in lib/rack/server.rb

If I traced right, this method called by build_app which definded in lib/rack/server.rb ( Rack )

def build_app(app)
  middleware[options[:environment]].reverse_each do |middleware|
    middleware = middleware.call(self) if middleware.respond_to?(:call)
    next unless middleware
    klass, *args = middleware
    app = klass.new(app, *args)
  end
  app
end

My question is: How does the middleware method works?

Answer 1

The method does not just return a blank hash, despite appearances. The Hash.new constructor takes the argument and makes that the default value returned if the specified key is missing. Example:

h=Hash.new([1])
h[:missing_key]
=> [1]

The build_app method is calling middleware[options[:environment]] . so if the specified environment doesn't exist, it will at least get back a minimal set of middleware.