Reproducing issue #20204 in rails/rails (4.2.1)

config__routes.rb

Rails.application.routes.draw do
  resources :fruits do
    mount TestEngine::Engine => '/test', as: :test_1
  end

  resources :vegetables do
    mount TestEngine::Engine => '/test', as: :test_2
  end
end

lib__engines__test_engine__config__routes.rb

TestEngine::Engine.routes.draw do
  scope module: "test" do
    root to: "tests#index"
    resources :tests
  end
end

name-routes-test-in-rails-console.log

leandroico:engine-routes-test leandro$ rails c
Warning: You're using Rubygems 2.0.14 with Spring. Upgrade to at least Rubygems 2.1.0 and run `gem pristine --all` for better startup performance.
Loading development environment (Rails 4.2.1)
2.1.5 :001 > app.fruit_test_1.tests_path(fruit_id: 1)
ActionController::UrlGenerationError: No route matches {:action=>"index"} missing required keys: [:vegetable_id]
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/journey/formatter.rb:46:in `generate'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:727:in `generate'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:758:in `generate'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:801:in `url_for'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:280:in `call'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:223:in `call'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:345:in `block (2 levels) in define_url_helper'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/mapper.rb:637:in `block (2 levels) in define_generate_prefix'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:792:in `url_for'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:280:in `call'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:223:in `call'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/route_set.rb:345:in `block (2 levels) in define_url_helper'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/routes_proxy.rb:31:in `tests_path'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.1/lib/action_dispatch/routing/routes_proxy.rb:34:in `method_missing'
	from (irb):1
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/railties-4.2.1/lib/rails/commands/console.rb:110:in `start'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/railties-4.2.1/lib/rails/commands/console.rb:9:in `start'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/railties-4.2.1/lib/rails/commands/commands_tasks.rb:68:in `console'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/railties-4.2.1/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/railties-4.2.1/lib/rails/commands.rb:17:in `<top (required)>'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `require'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `block in require'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:240:in `load_dependency'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `require'
	from /Users/leandro/projetos/engine-routes-test/bin/rails:8:in `<top (required)>'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:268:in `load'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:268:in `block in load'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:240:in `load_dependency'
	from /Users/leandro/.rvm/gems/ruby-2.1.5/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:268:in `load'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from -e:1:in `<main>'2.1.5 :002 >
2.1.5 :003 >   app.vegetable_test_2.tests_path(vegetable_id: 1)
 => "/vegetables/1/test/tests"
2.1.5 :004 > app.fruit_test_1.tests_path(vegetable_id: 1)
 => "/vegetables/1/test/tests"

rake-routes-output.log

leandroico:engine-routes-test leandro$ rake routes
          Prefix Verb   URI Pattern                    Controller#Action
    fruit_test_1        /fruits/:fruit_id/test         TestEngine::Engine
          fruits GET    /fruits(.:format)              fruits#index
                 POST   /fruits(.:format)              fruits#create
       new_fruit GET    /fruits/new(.:format)          fruits#new
      edit_fruit GET    /fruits/:id/edit(.:format)     fruits#edit
           fruit GET    /fruits/:id(.:format)          fruits#show
                 PATCH  /fruits/:id(.:format)          fruits#update
                 PUT    /fruits/:id(.:format)          fruits#update
                 DELETE /fruits/:id(.:format)          fruits#destroy
vegetable_test_2        /vegetables/:vegetable_id/test TestEngine::Engine
      vegetables GET    /vegetables(.:format)          vegetables#index
                 POST   /vegetables(.:format)          vegetables#create
   new_vegetable GET    /vegetables/new(.:format)      vegetables#new
  edit_vegetable GET    /vegetables/:id/edit(.:format) vegetables#edit
       vegetable GET    /vegetables/:id(.:format)      vegetables#show
                 PATCH  /vegetables/:id(.:format)      vegetables#update
                 PUT    /vegetables/:id(.:format)      vegetables#update
                 DELETE /vegetables/:id(.:format)      vegetables#destroy

Routes for TestEngine::Engine:
     root GET    /                         test_engine/test/tests#index
    tests GET    /tests(.:format)          test_engine/test/tests#index
          POST   /tests(.:format)          test_engine/test/tests#create
 new_test GET    /tests/new(.:format)      test_engine/test/tests#new
edit_test GET    /tests/:id/edit(.:format) test_engine/test/tests#edit
     test GET    /tests/:id(.:format)      test_engine/test/tests#show
          PATCH  /tests/:id(.:format)      test_engine/test/tests#update
          PUT    /tests/:id(.:format)      test_engine/test/tests#update
          DELETE /tests/:id(.:format)      test_engine/test/tests#destroy