Snippet from ActionDispatch::Routing::Mapper from Rails 4.2.5.2

mapper.rb

module ActionDispatch
  module Routing
    class Mapper
      URL_OPTIONS = [:protocol, :subdomain, :domain, :host, :port]
      
      class Constraints
        # code here
      end
      
      class Mapping
        # code here
      end
      
      module Base
        # Mount a Rack-based application to be used within the application.
        #
        #   mount SomeRackApp, at: "some_route"
        #
        # Alternatively:
        #
        #   mount(SomeRackApp => "some_route")
        #
        # For options, see +match+, as +mount+ uses it internally.
        #
        # All mounted applications come with routing helpers to access them.
        # These are named after the class specified, so for the above example
        # the helper is either +some_rack_app_path+ or +some_rack_app_url+.
        # To customize this helper's name, use the +:as+ option:
        #
        #   mount(SomeRackApp => "some_route", as: "exciting")
        #
        # This will generate the +exciting_path+ and +exciting_url+ helpers
        # which can be used to navigate to this mounted app.
        def mount(app, options = nil)
          if options
            path = options.delete(:at)
          else
            unless Hash === app
              raise ArgumentError, "must be called with mount point"
            end

            options = app
            app, path = options.find { |k, _| k.respond_to?(:call) }
            options.delete(app) if app
          end

          raise "A rack application must be specified" unless path

          rails_app = rails_app? app
          options[:as] ||= app_name(app, rails_app)

          target_as       = name_for_action(options[:as], path)
          options[:via] ||= :all

          match(path, options.merge(:to => app, :anchor => false, :format => false))

          define_generate_prefix(app, target_as) if rails_app
          self
        end
        
        
        private
          def rails_app?(app)
            app.is_a?(Class) && app < Rails::Railtie
          end

          def app_name(app, rails_app)
            if rails_app
              app.railtie_name
            elsif app.is_a?(Class)
              class_name = app.name
              ActiveSupport::Inflector.underscore(class_name).tr("/", "_")
            end
          end

          def define_generate_prefix(app, name)
            _route = @set.named_routes.get name
            _routes = @set
            app.routes.define_mounted_helper(name)
            app.routes.extend Module.new {
              def optimize_routes_generation?; false; end
              define_method :find_script_name do |options|
                if options.key? :script_name
                  super(options)
                else
                  prefix_options = options.slice(*_route.segment_keys)
                  prefix_options[:relative_url_root] = ''.freeze
                  # we must actually delete prefix segment keys to avoid passing them to next url_for
                  _route.segment_keys.each { |k| options.delete(k) }
                  _routes.url_helpers.send("#{name}_path", prefix_options)
                end
              end
            }
          end
      end
    end
  end