diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 2117cb76b5d9a25d6b37c32d784ca22c7e04a88f..ce4d4072172fddab6c870e2fa9b351b0d39f3716 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -16,7 +16,7 @@ def self.new(app, constraints, request = Rack::Request)
           end
         end
 
-        attr_reader :app
+        attr_reader :app, :constraints
 
         def initialize(app, constraints, request)
           @app, @constraints, @request = app, constraints, request
diff --git a/railties/lib/rails/application/route_inspector.rb b/railties/lib/rails/application/route_inspector.rb
index 26652a8e5e998600680027274e7079804dc835b6..3e08b782213db4acef8c38d164e0ff2cd9e18475 100644
--- a/railties/lib/rails/application/route_inspector.rb
+++ b/railties/lib/rails/application/route_inspector.rb
@@ -23,7 +23,7 @@ def collect_routes(routes)
         routes = routes.collect do |route|
           route_reqs = route.requirements
 
-          rack_app = route.app unless route.app.class.name.to_s =~ /^ActionDispatch::Routing/
+          rack_app = discover_rack_app(route.app)
 
           controller = route_reqs[:controller] || ':controller'
           action     = route_reqs[:action]     || ':action'
@@ -70,6 +70,15 @@ def formatted_routes(routes)
           "#{r[:name].rjust(name_width)} #{r[:verb].ljust(verb_width)} #{r[:path].ljust(path_width)} #{r[:reqs]}"
         end
       end
+
+      def discover_rack_app(app)
+        class_name = app.class.name.to_s
+        if class_name == "ActionDispatch::Routing::Mapper::Constraints"
+          discover_rack_app(app.app)
+        elsif class_name !~ /^ActionDispatch::Routing/
+          app
+        end
+      end
     end
   end
 end
diff --git a/railties/test/application/route_inspect_test.rb b/railties/test/application/route_inspect_test.rb
index 2ad5ee6c4c2656012c3277263c3e4baaa1825f8c..6503251b9fda178729cd0032548d61dca19333e1 100644
--- a/railties/test/application/route_inspect_test.rb
+++ b/railties/test/application/route_inspect_test.rb
@@ -127,5 +127,22 @@ def test_rake_routes_shows_route_with_rack_app
       output = @inspector.format @set.routes
       assert_equal ["  /foo/:id(.:format) #{RackApp.name} {:id=>/[A-Z]\\d{5}/}"], output
     end
+
+    def test_rake_routes_shows_route_with_rack_app_nested_with_dynamic_constraints
+      constraint = Class.new do
+        def to_s
+          "( my custom constraint )"
+        end
+      end
+
+      @set.draw do
+        scope :constraint => constraint.new do
+          mount RackApp => '/foo'
+        end
+      end
+
+      output = @inspector.format @set.routes
+      assert_equal ["  /foo #{RackApp.name} {:constraint=>( my custom constraint )}"], output
+    end
   end
 end