title: Adding Scope to ActiveAdmin routes getting ActionController::UrlGenerationError
body: <p>I'm attempting to add a scope to the ActiveAdmin routes so that I can include the '/:brand' within the URL.  SO instead of <code>localhost:3000/admin/dashboard</code> it would be <code>localhost:3000/bg/admin/dashboard</code>, with the bg being the brand.</p>
  
  <p>I have my routes.rb setup like this:</p>
  
  <pre><code>Rails.application.routes.draw do
    scope '/:brand', brand: /bg|gwt|acf/ do
      devise_for :admin_users, ActiveAdmin::Devise.config
      ActiveAdmin.routes(self)
    end
  end
  </code></pre>
  
  <p>The route for logging in <code>http://localhost:3000/bg/admin/login</code> works perfectly fine.  But once logged in I'm getting the below error for any ActiveAdmin resource.</p>
  
  <pre><code>ActionController::UrlGenerationError (No route matches {:action=&gt;"index", :controller=&gt;"admin/dashboard"} missing required keys: [:brand]):
      actionpack (5.0.0.1) lib/action_dispatch/journey/formatter.rb:50:in `generate'
  actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:629:in `generate'
  actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:660:in `generate'
  actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:707:in `url_for'
  actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:236:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:295:in `block (2 levels) in define_url_helper'
  /Users/computer/.rvm/gems/ruby-2.3.1/bundler/gems/activeadmin-f8926831429f/lib/active_admin/devise.rb:40:in `root_path'
  devise (4.2.0) lib/devise/controllers/helpers.rb:178:in `signed_in_root_path'
  devise (4.2.0) lib/devise/controllers/helpers.rb:214:in `after_sign_in_path_for'
  devise (4.2.0) app/controllers/devise_controller.rb:114:in `require_no_authentication'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:382:in `block in make_lambda'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:150:in `block (2 levels) in halting_and_conditional'
  actionpack (5.0.0.1) lib/abstract_controller/callbacks.rb:12:in `block (2 levels) in &lt;module:Callbacks&gt;'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:151:in `block in halting_and_conditional'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:454:in `block in call'
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  activesupport (5.0.0.1) lib/active_support/callbacks.rb:454:in `call'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:101:in `__run_callbacks__'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:750:in `_run_process_action_callbacks'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:90:in `run_callbacks'
  actionpack (5.0.0.1) lib/abstract_controller/callbacks.rb:19:in `process_action'
  actionpack (5.0.0.1) lib/action_controller/metal/rescue.rb:20:in `process_action'
  actionpack (5.0.0.1) lib/action_controller/metal/instrumentation.rb:32:in `block in process_action'
  activesupport (5.0.0.1) lib/active_support/notifications.rb:164:in `block in instrument'
  activesupport (5.0.0.1) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
  activesupport (5.0.0.1) lib/active_support/notifications.rb:164:in `instrument'
  actionpack (5.0.0.1) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
  actionpack (5.0.0.1) lib/action_controller/metal/params_wrapper.rb:248:in `process_action'
  activerecord (5.0.0.1) lib/active_record/railties/controller_runtime.rb:18:in `process_action'
  actionpack (5.0.0.1) lib/abstract_controller/base.rb:126:in `process'
  actionview (5.0.0.1) lib/action_view/rendering.rb:30:in `process'
  actionpack (5.0.0.1) lib/action_controller/metal.rb:190:in `dispatch'
  actionpack (5.0.0.1) lib/action_controller/metal.rb:262:in `dispatch'
  actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:50:in `dispatch'
  actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:32:in `serve'
  actionpack (5.0.0.1) lib/action_dispatch/routing/mapper.rb:16:in `block in &lt;class:Constraints&gt;'
  actionpack (5.0.0.1) lib/action_dispatch/routing/mapper.rb:46:in `serve'
  actionpack (5.0.0.1) lib/action_dispatch/journey/router.rb:39:in `block in serve'
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  actionpack (5.0.0.1) lib/action_dispatch/journey/router.rb:26:in `serve'
  actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:725:in `call'
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  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/rack/browser_monitoring.rb:32:in `traced_call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/rack/developer_mode.rb:48:in `traced_call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  warden (1.2.6) lib/warden/manager.rb:35:in `block in call'
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  warden (1.2.6) lib/warden/manager.rb:34:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  rack (2.0.1) lib/rack/etag.rb:25:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  rack (2.0.1) lib/rack/conditional_get.rb:25:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  rack (2.0.1) lib/rack/head.rb:12:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  rack (2.0.1) lib/rack/session/abstract/id.rb:222:in `context'
  rack (2.0.1) lib/rack/session/abstract/id.rb:216:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/cookies.rb:613:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  activerecord (5.0.0.1) lib/active_record/migration.rb:552:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/callbacks.rb:38:in `block in call'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:97:in `__run_callbacks__'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:750:in `_run_call_callbacks'
  activesupport (5.0.0.1) lib/active_support/callbacks.rb:90:in `run_callbacks'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/callbacks.rb:36:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/executor.rb:12:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/remote_ip.rb:79:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/debug_exceptions.rb:49:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  web-console (3.3.1) lib/web_console/middleware.rb:131:in `call_app'
  web-console (3.3.1) lib/web_console/middleware.rb:28:in `block in call'
  web-console (3.3.1) lib/web_console/middleware.rb:18:in `catch'
  web-console (3.3.1) lib/web_console/middleware.rb:18:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/show_exceptions.rb:31:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  railties (5.0.0.1) lib/rails/rack/logger.rb:36:in `call_app'
  railties (5.0.0.1) lib/rails/rack/logger.rb:24:in `block in call'
  activesupport (5.0.0.1) lib/active_support/tagged_logging.rb:70:in `block in tagged'
  activesupport (5.0.0.1) lib/active_support/tagged_logging.rb:26:in `tagged'
  activesupport (5.0.0.1) lib/active_support/tagged_logging.rb:70:in `tagged'
  railties (5.0.0.1) lib/rails/rack/logger.rb:24:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  sprockets-rails (3.2.0) lib/sprockets/rails/quiet_assets.rb:13:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/request_id.rb:24:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  rack (2.0.1) lib/rack/method_override.rb:22:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  rack (2.0.1) lib/rack/runtime.rb:22:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  rack-timeout (0.4.2) lib/rack/timeout/core.rb:122:in `block in call'
  rack-timeout (0.4.2) lib/rack/timeout/support/timeout.rb:19:in `timeout'
  rack-timeout (0.4.2) lib/rack/timeout/core.rb:121:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  activesupport (5.0.0.1) lib/active_support/cache/strategy/local_cache_middleware.rb:28:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/executor.rb:12:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  actionpack (5.0.0.1) lib/action_dispatch/middleware/static.rb:136:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  rack-cors (0.4.0) lib/rack/cors.rb:80:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  rack (2.0.1) lib/rack/sendfile.rb:111:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  railties (5.0.0.1) lib/rails/engine.rb:522:in `call'
  newrelic_rpm (3.15.2.317) lib/new_relic/agent/instrumentation/middleware_tracing.rb:96:in `call'
  puma (3.6.0) lib/puma/configuration.rb:225:in `call'
  puma (3.6.0) lib/puma/server.rb:578:in `handle_request'
  puma (3.6.0) lib/puma/server.rb:415:in `process_client'
  puma (3.6.0) lib/puma/server.rb:275:in `block in run'
  puma (3.6.0) lib/puma/thread_pool.rb:116:in `block in spawn_thread'
    Rendering /Users/computer/.rvm/gems/ruby-2.3.1/gems/actionpack-5.0.0.1/lib/action_dispatch/middleware/templates/rescues/diagnostics.html.erb within rescues/layout
    Rendering /Users/computer/.rvm/gems/ruby-2.3.1/gems/actionpack-5.0.0.1/lib/action_dispatch/middleware/templates/rescues/_source.html.erb
    Rendered /Users/computer/.rvm/gems/ruby-2.3.1/gems/actionpack-5.0.0.1/lib/action_dispatch/middleware/templates/rescues/_source.html.erb (4.3ms)
    Rendering /Users/computer/.rvm/gems/ruby-2.3.1/gems/actionpack-5.0.0.1/lib/action_dispatch/middleware/templates/rescues/_trace.html.erb
    Rendered /Users/computer/.rvm/gems/ruby-2.3.1/gems/actionpack-5.0.0.1/lib/action_dispatch/middleware/templates/rescues/_trace.html.erb (1.7ms)
    Rendering /Users/computer/.rvm/gems/ruby-2.3.1/gems/actionpack-5.0.0.1/lib/action_dispatch/middleware/templates/rescues/_request_and_response.html.erb
    Rendered /Users/computer/.rvm/gems/ruby-2.3.1/gems/actionpack-5.0.0.1/lib/action_dispatch/middleware/templates/rescues/_request_and_response.html.erb (1.0ms)
    Rendered /Users/computer/.rvm/gems/ruby-2.3.1/gems/actionpack-5.0.0.1/lib/action_dispatch/middleware/templates/rescues/diagnostics.html.erb within rescues/layout (103.9ms)
  source=rack-timeout id=da53bfbfe078024cf2939732b0756051 timeout=3100000ms service=578ms state=completed
  </code></pre>
  
  <p>Anyone have any idea why this is happening or a way to get around it?  Any help is greatly appreciated.  Thanks!</p>