Ruby on Rails NoMethodError：nil：NilClass的未定义方法“ []”

Question

I'm new to ruby on rails and currently working on a web application which uses the 'rspotify' gem to interact with the Spotify API.

I have a controller called 'genre' which I use to direct my search results from the index page. However, it won't let me initialize any variables inside the index function.

This is what my controller looks like

# genre_controller.rb
  def index
    if params[:search]
      @genres = RSpotify::Recommendations.generate(limit: 20, seed_genres: ['country'])
      # render :json => @songs
    else
      redirect_to root_path
    end
  end

This is what my my index page looks like

<h1>Genre#index</h1>
<p>Find me in app/views/genre/index.html.erb</p>

<h1 class="page-header">Music Matching <%= params[:search] %></h1>
<ul>
 <% @music.each do |music| %>
   <li><%= music.name %> | <%= link_to "Show", genre_path(music.id) %></li>
 <% end %>
</ul>

This is what my routes look like

  get 'genre/index'
  get 'genre/show', to:'genre#show', as: 'genre'

  get 'site/index'
  root 'site#index'

I get an error on line 4 of my genre controller, where I call the RSpotify method. Any variable I try to instantiate there returns a NoErrorMethod.

Any help is appreciated, thanks!

Edit:

Here's the exception I get from the console

NoMethodError (undefined method `[]' for nil:NilClass):

app/controllers/genre_controller.rb:4:in `index'
Started GET "/genre/index?utf8=%E2%9C%93&search=hello" for 127.0.0.1 at 2018-02-19 16:14:59 -0800
Processing by GenreController#index as HTML
  Parameters: {"utf8"=>"✓", "search"=>"hello"}
Completed 500 Internal Server Error in 295ms

Here's the full tracestack from the exception

rspotify (1.27.0) lib/rspotify/recommendations.rb:101:in `initialize'
rspotify (1.27.0) lib/rspotify/recommendations.rb:97:in `new'
rspotify (1.27.0) lib/rspotify/recommendations.rb:97:in `generate'
app/controllers/genre_controller.rb:4:in `index'
actionpack (5.1.4) lib/action_controller/metal/basic_implicit_render.rb:4:in `send_action'
actionpack (5.1.4) lib/abstract_controller/base.rb:186:in `process_action'
actionpack (5.1.4) lib/action_controller/metal/rendering.rb:30:in `process_action'
actionpack (5.1.4) lib/abstract_controller/callbacks.rb:20:in `block in process_action'
activesupport (5.1.4) lib/active_support/callbacks.rb:131:in `run_callbacks'
actionpack (5.1.4) lib/abstract_controller/callbacks.rb:19:in `process_action'
actionpack (5.1.4) lib/action_controller/metal/rescue.rb:20:in `process_action'
actionpack (5.1.4) lib/action_controller/metal/instrumentation.rb:32:in `block in process_action'
activesupport (5.1.4) lib/active_support/notifications.rb:166:in `block in instrument'
activesupport (5.1.4) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
activesupport (5.1.4) lib/active_support/notifications.rb:166:in `instrument'
actionpack (5.1.4) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
actionpack (5.1.4) lib/action_controller/metal/params_wrapper.rb:252:in `process_action'
activerecord (5.1.4) lib/active_record/railties/controller_runtime.rb:22:in `process_action'
actionpack (5.1.4) lib/abstract_controller/base.rb:124:in `process'
actionview (5.1.4) lib/action_view/rendering.rb:30:in `process'
actionpack (5.1.4) lib/action_controller/metal.rb:189:in `dispatch'
actionpack (5.1.4) lib/action_controller/metal.rb:253:in `dispatch'
actionpack (5.1.4) lib/action_dispatch/routing/route_set.rb:49:in `dispatch'
actionpack (5.1.4) lib/action_dispatch/routing/route_set.rb:31:in `serve'
actionpack (5.1.4) lib/action_dispatch/journey/router.rb:50:in `block in serve'
actionpack (5.1.4) lib/action_dispatch/journey/router.rb:33:in `each'
actionpack (5.1.4) lib/action_dispatch/journey/router.rb:33:in `serve'
actionpack (5.1.4) lib/action_dispatch/routing/route_set.rb:834:in `call'
rack (2.0.4) lib/rack/etag.rb:25:in `call'
rack (2.0.4) lib/rack/conditional_get.rb:25:in `call'
rack (2.0.4) lib/rack/head.rb:12:in `call'
rack (2.0.4) lib/rack/session/abstract/id.rb:232:in `context'
rack (2.0.4) lib/rack/session/abstract/id.rb:226:in `call'
actionpack (5.1.4) lib/action_dispatch/middleware/cookies.rb:613:in `call'
activerecord (5.1.4) lib/active_record/migration.rb:556:in `call'
actionpack (5.1.4) lib/action_dispatch/middleware/callbacks.rb:26:in `block in call'
activesupport (5.1.4) lib/active_support/callbacks.rb:97:in `run_callbacks'
actionpack (5.1.4) lib/action_dispatch/middleware/callbacks.rb:24:in `call'
actionpack (5.1.4) lib/action_dispatch/middleware/executor.rb:12:in `call'
actionpack (5.1.4) lib/action_dispatch/middleware/debug_exceptions.rb:59:in `call'
web-console (3.5.1) lib/web_console/middleware.rb:135:in `call_app'
web-console (3.5.1) lib/web_console/middleware.rb:28:in `block in call'
web-console (3.5.1) lib/web_console/middleware.rb:18:in `catch'
web-console (3.5.1) lib/web_console/middleware.rb:18:in `call'
actionpack (5.1.4) lib/action_dispatch/middleware/show_exceptions.rb:31:in `call'
railties (5.1.4) lib/rails/rack/logger.rb:36:in `call_app'
railties (5.1.4) lib/rails/rack/logger.rb:24:in `block in call'
activesupport (5.1.4) lib/active_support/tagged_logging.rb:69:in `block in tagged'
activesupport (5.1.4) lib/active_support/tagged_logging.rb:26:in `tagged'
activesupport (5.1.4) lib/active_support/tagged_logging.rb:69:in `tagged'
railties (5.1.4) lib/rails/rack/logger.rb:24:in `call'
sprockets-rails (3.2.1) lib/sprockets/rails/quiet_assets.rb:13:in `call'
actionpack (5.1.4) lib/action_dispatch/middleware/remote_ip.rb:79:in `call'
actionpack (5.1.4) lib/action_dispatch/middleware/request_id.rb:25:in `call'
rack (2.0.4) lib/rack/method_override.rb:22:in `call'
rack (2.0.4) lib/rack/runtime.rb:22:in `call'
activesupport (5.1.4) lib/active_support/cache/strategy/local_cache_middleware.rb:27:in `call'
actionpack (5.1.4) lib/action_dispatch/middleware/executor.rb:12:in `call'
actionpack (5.1.4) lib/action_dispatch/middleware/static.rb:125:in `call'
rack (2.0.4) lib/rack/sendfile.rb:111:in `call'
railties (5.1.4) lib/rails/engine.rb:522:in `call'
puma (3.11.2) lib/puma/configuration.rb:225:in `call'
puma (3.11.2) lib/puma/server.rb:624:in `handle_request'
puma (3.11.2) lib/puma/server.rb:438:in `process_client'
puma (3.11.2) lib/puma/server.rb:302:in `block in run'
puma (3.11.2) lib/puma/thread_pool.rb:120:in `block in spawn_thread'

Answer 1

I got a similar error as above when I tried to connect to the Spotify API. The documentation on rspotify missed out the fact that you need to run the authenticate method before you do anything with the API.

ie

RSpotify.authenticate("<your_client_id>", "<your_client_secret>")