(course)new.html.erb

<% provide(:title, "Create course") %>

<div class="container">

  <div class="row">
    <div class="col-md-4">
      <div class="custom-form chapter">
        <%= form_for(@course) do |f| %>
          <%= render 'shared/error_messages', object: f.object %>
          <%= f.label :title %>
          <%= f.text_field :title, class: 'form-control' %>

          <%= f.label :description %>
          <%= f.text_area :description, class: 'form-control', :cols => "10", :rows => "10" %>

          <%= f.label :price %>
          <%= f.number_field :price, class: 'form-control' %>
          <%= f.submit "Save", class: "btn btn-large btn-primary btn-block btn-course" %>
        <% end %>
      </div>

        
    </div>

    <% if @course.persisted? %>
      <div class="col-md-8">
        <ul class="nav nav-tabs">
          <li class="active"><a href="#">Chapter 1</a></li>
          <li><a href="#">New Chapter</a></li>
        </ul>
        <%= link_to "New Chapter", new_course_chapter_path(@course) %>
      </div>
    <% end %>
  </div>
</div>

chapter.rb

class Chapter < ActiveRecord::Base
  belongs_to :course

  default_scope -> { order("number ASC") }
  validates :title, presence: true, length: { maximum: 80 }
  validates :number, presence: true
  validates :course_id, presence: true
end

chapters_controller.rb

class ChaptersController < ApplicationController
  before_action :set_course
  before_action :set_chapter, only: [:show, :edit, :update, :destroy]
  
  before_action :signed_in_user, only: [:create, :destroy]

  def index
  end

  def new
    @chapter = @course.chapters.build
  end

  def show
  end

  def create
    @chapter = @course.chapters.build(chapter_params)
    if @chapter.save
      flash[:success] = "Chapter created! :)"
      redirect_to [@course, @chapter]
    else
      flash[:danger] = "Chapter has not been created! :("
      render 'new'
    end
  end

  def destroy
  end

private
  def chapter_params
    params.require(:chapter).permit(:title, :number)
  end

  def set_course
    @course = Course.find(params[:course_id])
  end

  def set_chapter
    @chapter = @course.chapters.find(params[:id])
  end
end

course.rb

class Course < ActiveRecord::Base
  belongs_to :user
  has_many :chapters, dependent: :destroy

  default_scope -> { order("created_at DESC") }
  validates :user_id, presence: true
  validates :title, presence: true, length: { maximum: 80 }
  validates :description, presence: true, length: { maximum: 350 }
  validates :price, presence: true
end

courses_controller.rb

class CoursesController < ApplicationController
  before_action :signed_in_user, only: [:create, :destroy]
  before_action :correct_user, only: :destroy
  
  before_action :set_course, only: [:show, :edit, :update, :destroy]

  def index
    @courses = Course.all
  end

  def new
    @course = Course.new
    @chapter = @course.chapters.build
  end

  def show
    @chapters = @course.chapters(page: params[:page])
  end

  def create
    @course = current_user.courses.build(course_params)
    if @course.save
      flash[:success] = "Course created! :)"
      redirect_to @course
    else
      flash[:danger] = "Course has not been created! :("
      render 'new'
    end
  end

  def destroy
    @course.destroy
    flash[:success] = "Course deleted!"
    redirect_to current_user
  end

private
  def course_params
    params.require(:course).permit(:title, :description, :price)
  end

  def correct_user
    @course = current_user.courses.find_by(id: params[:id])
    redirect_to current_user if @course.nil?
  end

  def set_course
    @course = Course.find(params[:id])
  rescue ActiveRecord::RecordNotFound
    flash[:danger] = "The course you were looking" +
                    " for could not be found."

    redirect_to courses_path
  end

end

error

ActionController::UrlGenerationError in Courses#new
Showing /Users/juanmnl/code/rails/elearning/app/views/courses/new.html.erb where line #30 raised:

No route matches {:course_id=>#<Course id: nil, title: nil, description: nil, price: nil, user_id: nil, rating: nil, created_at: nil, updated_at: nil, published: nil>} missing required keys: [:course_id]
Extracted source (around line #30):

           <li class="active"><a href="#">Chapter 1</a></li>
           <li><a href="#">New Chapter</a></li>
         </ul>
         <%= link_to "New Chapter", new_course_chapter_path(@course) %>
       </div>
     </div>
   </div>
   
/////////// FULL TRACE
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:223:in `raise_generation_error'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:192:in `block in optimized_helper'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:188:in `zip'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:188:in `optimized_helper'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:175:in `call'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:274:in `block (2 levels) in define_url_helper'
app/views/courses/new.html.erb:30:in `_app_views_courses_new_html_erb___217999686602933813_70155231674840'
actionpack (4.0.2) lib/action_view/template.rb:143:in `block in render'
activesupport (4.0.2) lib/active_support/notifications.rb:161:in `instrument'
actionpack (4.0.2) lib/action_view/template.rb:141:in `render'
actionpack (4.0.2) lib/action_view/renderer/template_renderer.rb:49:in `block (2 levels) in render_template'
actionpack (4.0.2) lib/action_view/renderer/abstract_renderer.rb:38:in `block in instrument'
activesupport (4.0.2) lib/active_support/notifications.rb:159:in `block in instrument'
activesupport (4.0.2) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
activesupport (4.0.2) lib/active_support/notifications.rb:159:in `instrument'
actionpack (4.0.2) lib/action_view/renderer/abstract_renderer.rb:38:in `instrument'
actionpack (4.0.2) lib/action_view/renderer/template_renderer.rb:48:in `block in render_template'
actionpack (4.0.2) lib/action_view/renderer/template_renderer.rb:56:in `render_with_layout'
actionpack (4.0.2) lib/action_view/renderer/template_renderer.rb:47:in `render_template'
actionpack (4.0.2) lib/action_view/renderer/template_renderer.rb:17:in `render'
actionpack (4.0.2) lib/action_view/renderer/renderer.rb:42:in `render_template'
actionpack (4.0.2) lib/action_view/renderer/renderer.rb:23:in `render'
actionpack (4.0.2) lib/abstract_controller/rendering.rb:127:in `_render_template'
actionpack (4.0.2) lib/action_controller/metal/streaming.rb:219:in `_render_template'
actionpack (4.0.2) lib/abstract_controller/rendering.rb:120:in `render_to_body'
actionpack (4.0.2) lib/action_controller/metal/rendering.rb:33:in `render_to_body'
actionpack (4.0.2) lib/action_controller/metal/renderers.rb:26:in `render_to_body'
actionpack (4.0.2) lib/abstract_controller/rendering.rb:97:in `render'
actionpack (4.0.2) lib/action_controller/metal/rendering.rb:16:in `render'
actionpack (4.0.2) lib/action_controller/metal/instrumentation.rb:41:in `block (2 levels) in render'
activesupport (4.0.2) lib/active_support/core_ext/benchmark.rb:12:in `block in ms'
/Users/juanmnl/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/benchmark.rb:294:in `realtime'
activesupport (4.0.2) lib/active_support/core_ext/benchmark.rb:12:in `ms'
actionpack (4.0.2) lib/action_controller/metal/instrumentation.rb:41:in `block in render'
actionpack (4.0.2) lib/action_controller/metal/instrumentation.rb:84:in `cleanup_view_runtime'
activerecord (4.0.2) lib/active_record/railties/controller_runtime.rb:25:in `cleanup_view_runtime'
actionpack (4.0.2) lib/action_controller/metal/instrumentation.rb:40:in `render'
actionpack (4.0.2) lib/action_controller/metal/implicit_render.rb:10:in `default_render'
actionpack (4.0.2) lib/action_controller/metal/implicit_render.rb:5:in `send_action'
actionpack (4.0.2) lib/abstract_controller/base.rb:189:in `process_action'
actionpack (4.0.2) lib/action_controller/metal/rendering.rb:10:in `process_action'
actionpack (4.0.2) lib/abstract_controller/callbacks.rb:18:in `block in process_action'
activesupport (4.0.2) lib/active_support/callbacks.rb:433:in `_run__4248797210385187787__process_action__callbacks'
activesupport (4.0.2) lib/active_support/callbacks.rb:80:in `run_callbacks'
actionpack (4.0.2) lib/abstract_controller/callbacks.rb:17:in `process_action'
actionpack (4.0.2) lib/action_controller/metal/rescue.rb:29:in `process_action'
actionpack (4.0.2) lib/action_controller/metal/instrumentation.rb:31:in `block in process_action'
activesupport (4.0.2) lib/active_support/notifications.rb:159:in `block in instrument'
activesupport (4.0.2) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
activesupport (4.0.2) lib/active_support/notifications.rb:159:in `instrument'
actionpack (4.0.2) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
actionpack (4.0.2) lib/action_controller/metal/params_wrapper.rb:245:in `process_action'
activerecord (4.0.2) lib/active_record/railties/controller_runtime.rb:18:in `process_action'
actionpack (4.0.2) lib/abstract_controller/base.rb:136:in `process'
actionpack (4.0.2) lib/abstract_controller/rendering.rb:44:in `process'
actionpack (4.0.2) lib/action_controller/metal.rb:195:in `dispatch'
actionpack (4.0.2) lib/action_controller/metal/rack_delegation.rb:13:in `dispatch'
actionpack (4.0.2) lib/action_controller/metal.rb:231:in `block in action'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:80:in `call'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:80:in `dispatch'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:48:in `call'
actionpack (4.0.2) lib/action_dispatch/journey/router.rb:71:in `block in call'
actionpack (4.0.2) lib/action_dispatch/journey/router.rb:59:in `each'
actionpack (4.0.2) lib/action_dispatch/journey/router.rb:59:in `call'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:680:in `call'
rack (1.5.2) lib/rack/etag.rb:23:in `call'
rack (1.5.2) lib/rack/conditionalget.rb:25:in `call'
rack (1.5.2) lib/rack/head.rb:11:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/params_parser.rb:27:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/flash.rb:241:in `call'
rack (1.5.2) lib/rack/session/abstract/id.rb:225:in `context'
rack (1.5.2) lib/rack/session/abstract/id.rb:220:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/cookies.rb:486:in `call'
activerecord (4.0.2) lib/active_record/query_cache.rb:36:in `call'
activerecord (4.0.2) lib/active_record/connection_adapters/abstract/connection_pool.rb:626:in `call'
activerecord (4.0.2) lib/active_record/migration.rb:369:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/callbacks.rb:29:in `block in call'
activesupport (4.0.2) lib/active_support/callbacks.rb:373:in `_run__2969075338233753350__call__callbacks'
activesupport (4.0.2) lib/active_support/callbacks.rb:80:in `run_callbacks'
actionpack (4.0.2) lib/action_dispatch/middleware/callbacks.rb:27:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/reloader.rb:64:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/remote_ip.rb:76:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/debug_exceptions.rb:17:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/show_exceptions.rb:30:in `call'
railties (4.0.2) lib/rails/rack/logger.rb:38:in `call_app'
railties (4.0.2) lib/rails/rack/logger.rb:20:in `block in call'
activesupport (4.0.2) lib/active_support/tagged_logging.rb:67:in `block in tagged'
activesupport (4.0.2) lib/active_support/tagged_logging.rb:25:in `tagged'
activesupport (4.0.2) lib/active_support/tagged_logging.rb:67:in `tagged'
railties (4.0.2) lib/rails/rack/logger.rb:20:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/request_id.rb:21:in `call'
rack (1.5.2) lib/rack/methodoverride.rb:21:in `call'
rack (1.5.2) lib/rack/runtime.rb:17:in `call'
activesupport (4.0.2) lib/active_support/cache/strategy/local_cache.rb:83:in `call'
rack (1.5.2) lib/rack/lock.rb:17:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/static.rb:64:in `call'
rack (1.5.2) lib/rack/sendfile.rb:112:in `call'
railties (4.0.2) lib/rails/engine.rb:511:in `call'
railties (4.0.2) lib/rails/application.rb:97:in `call'
rack (1.5.2) lib/rack/lock.rb:17:in `call'
rack (1.5.2) lib/rack/content_length.rb:14:in `call'
rack (1.5.2) lib/rack/handler/webrick.rb:60:in `service'
/Users/juanmnl/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/webrick/httpserver.rb:138:in `service'
/Users/juanmnl/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/webrick/httpserver.rb:94:in `run'
/Users/juanmnl/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/webrick/server.rb:295:in `block in start_thread'

routes.rb

Elearning::Application.routes.draw do
  
  resources :users
  resources :sessions, only: [:new, :create, :destroy]
  resources :courses do
    resources :chapters
  end
end