Use env['action_dispatch.routes'] to determine if we should generate prefix or not.

This technique is here to allow using routes from Engine in Application
and vice versa. When using Engine routes inside Application it should
generate prefix based on mount point. When using Engine routes inside
Engine it should use env['SCRIPT_NAME']. In any other case it should
generate prefix as env should not be even available.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -261,7 +261,11 @@ def mount(app, options = nil)
 
           raise "A rack application must be specified" unless path
 
+          options[:as] ||= app_name(app)
+
           match(path, options.merge(:to => app, :anchor => false, :format => false))
+
+          define_generate_prefix(app, options[:as])
           self
         end
 
@@ -269,6 +273,29 @@ def default_url_options=(options)
           @set.default_url_options = options
         end
         alias_method :default_url_options, :default_url_options=
+
+        private
+          def app_name(app)
+            return unless app.respond_to?(:routes)
+            class_name = app.class.is_a?(Class) ? app.name : app.class.name
+            ActiveSupport::Inflector.underscore(class_name).gsub("/", "_")
+          end
+
+          def define_generate_prefix(app, name)
+            return unless app.respond_to?(:routes)
+
+            _route = @set.named_routes.routes[name.to_sym]
+            _router = @set
+            app.routes.class_eval do
+              define_method :_generate_prefix do |options|
+                keys = _route.segment_keys + ActionDispatch::Routing::RouteSet::RESERVED_OPTIONS
+                prefix_options = options.reject { |k, v| !(keys).include?(k) }
+                # we must actually delete prefix segment keys to avoid passing them to next url_for
+                _route.segment_keys.each { |k| options.delete(k) }
+                _router.url_helpers.send("#{name}_path", prefix_options)
+              end
+            end
+          end
       end
 
       module HttpHelpers

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -303,10 +303,9 @@ def add_route(app, conditions = {}, requirements = {}, defaults = {}, name = nil
       end
 
       class Generator #:nodoc:
-        attr_reader :options, :recall, :set, :script_name, :named_route
+        attr_reader :options, :recall, :set, :named_route
 
         def initialize(options, recall, set, extras = false)
-          @script_name = options.delete(:script_name)
           @named_route = options.delete(:use_route)
           @options     = options.dup
           @recall      = recall.dup
@@ -401,7 +400,7 @@ def generate
           return [path, params.keys] if @extras
 
           path << "?#{params.to_query}" if params.any?
-          "#{script_name}#{path}"
+          path
         rescue Rack::Mount::RoutingError
           raise_routing_error
         end
@@ -453,7 +452,11 @@ def generate(options, recall = {}, extras = false)
         Generator.new(options, recall, self, extras).generate
       end
 
-      RESERVED_OPTIONS = [:anchor, :params, :only_path, :host, :protocol, :port, :trailing_slash]
+      RESERVED_OPTIONS = [:anchor, :params, :only_path, :host, :protocol, :port, :trailing_slash, :script_name, :skip_prefix, :routes]
+
+      def _generate_prefix(options = {})
+        nil
+      end
 
       def url_for(options)
         finalize!
@@ -464,7 +467,6 @@ def url_for(options)
         rewritten_url = ""
 
         path_segments = options.delete(:_path_segments)
-
         unless options[:only_path]
           rewritten_url << (options[:protocol] || "http")
           rewritten_url << "://" unless rewritten_url.match("://")
@@ -476,9 +478,15 @@ def url_for(options)
           rewritten_url << ":#{options.delete(:port)}" if options.key?(:port)
         end
 
+        path = [options.delete(:script_name)]
+        if !options.delete(:skip_prefix)
+          path << _generate_prefix(options)
+        end
+
         path_options = options.except(*RESERVED_OPTIONS)
         path_options = yield(path_options) if block_given?
-        path = generate(path_options, path_segments || {})
+        path << generate(path_options, path_segments || {})
+        path = path.compact.join ''
 
         # ROUTES TODO: This can be called directly, so script_name should probably be set in the routes
         rewritten_url << (options[:trailing_slash] ? path.sub(/\?|\z/) { "/" + $& } : path)

actionpack/lib/action_dispatch/routing/url_for.rb

@@ -128,7 +128,13 @@ def url_for(options = nil)
         when String
           options
         when nil, Hash
-          _routes.url_for((options || {}).reverse_merge!(url_options).symbolize_keys)
+          routes = (options ? options.delete(:routes) : nil) || _routes
+
+          if respond_to?(:env) && env && routes.equal?(env["action_dispatch.routes"])
+             options[:skip_prefix] = true
+          end
+
+          routes.url_for((options || {}).reverse_merge!(url_options).symbolize_keys)
         else
           polymorphic_url(options)
         end

actionpack/test/controller/routing_test.rb

@@ -251,7 +251,7 @@ def test_optimised_named_route_with_host
       map.pages 'pages', :controller => 'content', :action => 'show_page', :host => 'foo.com'
     end
     x = setup_for_named_route
-    x.expects(:url_for).with(:host => 'foo.com', :only_path => false, :controller => 'content', :action => 'show_page', :use_route => :pages).once
+    x.expects(:url_for).with(:host => 'foo.com', :only_path => false, :controller => 'content', :action => 'show_page', :use_route => :pages, :router => rs).once
     x.send(:pages_url)
   end
 

actionpack/test/dispatch/prefix_generation_test.rb

+require 'abstract_unit'
+
+module TestGenerationPrefix
+  class WithMountedEngine < ActionDispatch::IntegrationTest
+    class BlogEngine
+      def self.routes
+        @routes ||= begin
+          routes = ActionDispatch::Routing::RouteSet.new
+          routes.draw do
+            match "/posts/:id", :to => "inside_engine_generating#index", :as => :post
+          end
+
+          routes
+        end
+      end
+
+      def self.call(env)
+        env['action_dispatch.routes'] = routes
+        routes.call(env)
+      end
+    end
+
+    class RailsApplication
+      def self.routes
+        @routes ||= begin
+          routes = ActionDispatch::Routing::RouteSet.new
+          routes.draw do
+            scope "/:omg", :omg => "awesome" do
+              mount BlogEngine => "/blog"
+            end
+            match "/generate", :to => "outside_engine_generating#index"
+          end
+
+          routes
+        end
+      end
+
+      def self.call(env)
+        env['action_dispatch.routes'] = routes
+        routes.call(env)
+      end
+    end
+
+    class ::InsideEngineGeneratingController < ActionController::Base
+      include BlogEngine.routes.url_helpers
+      def index
+        render :text => post_path(:id => params[:id])
+      end
+    end
+
+    class ::OutsideEngineGeneratingController < ActionController::Base
+      include BlogEngine.routes.url_helpers
+      def index
+        render :text => post_path(:id => 1)
+      end
+    end
+
+    class Foo
+      include ActionDispatch::Routing::UrlFor
+      include BlogEngine.routes.url_helpers
+
+      def foo
+        post_path(42)
+      end
+    end
+
+
+    RailsApplication.routes # force draw
+    include BlogEngine.routes.url_helpers
+
+    test "generating URL with prefix" do
+      assert_equal "/awesome/blog/posts/1", post_path(:id => 1)
+    end
+
+    test "use SCRIPT_NAME inside the engine" do
+      env = Rack::MockRequest.env_for("/posts/1")
+      env["SCRIPT_NAME"] = "/pure-awesomness/blog"
+      response = ActionDispatch::Response.new(*BlogEngine.call(env))
+      assert_equal "/pure-awesomness/blog/posts/1", response.body
+    end
+
+    test "prepend prefix outside the engine" do
+      env = Rack::MockRequest.env_for("/generate")
+      env["SCRIPT_NAME"] = "/something" # it could be set by passenger
+      response = ActionDispatch::Response.new(*RailsApplication.call(env))
+      assert_equal "/something/awesome/blog/posts/1", response.body
+    end
+
+    test "generating urls with options for both prefix and named_route" do
+      assert_equal "/pure-awesomness/blog/posts/3", post_path(:id => 3, :omg => "pure-awesomness")
+    end
+
+    test "generating urls with url_for should prepend the prefix" do
+      path = BlogEngine.routes.url_for(:omg => 'omg', :controller => "inside_engine_generating", :action => "index", :id => 1, :only_path => true)
+      assert_equal "/omg/blog/posts/1", path
+    end
+
+    test "generating urls from a regular class" do
+      assert_equal "/awesome/blog/posts/42", Foo.new.foo
+    end
+  end
+end