Cybertrain::Routes.draw do
  root "articles#index"

  # A nested block takes the mapper explicitly (Spinel's instance_eval
  # trampoline only rewires `self` for the outermost block): `articles.resources`.
  resources :articles do |articles|
    articles.resources :comments, only: [:create, :destroy]
  end
end

# app/controllers/articles_controller.rb (index/show/edit are as plain as
# Rails' own scaffold; update mirrors create)
class ArticlesController < ApplicationController
  before_action :set_article, only: [:show, :edit, :update, :destroy]

  def new_action   # a `new` method would shadow ArticlesController.new
    @article = Article.new
  end

  def create
    @article = Article.new(article_params)
    if @article.save
      flash[:notice] = "Article was successfully created."
      redirect_to article_path(@article), status: :see_other
    else
      render :new, status: :unprocessable_entity
    end
  end

  private

  def set_article
    @article = Article.find(params[:id])
  end

  def article_params
    params.require(:article).permit(:title, :body)
  end
end

<%# locals: (comment:) %>
<div class="comment">
  <p>
    <strong>Commenter:</strong>
    <%= comment.commenter %>
  </p>

  <p>
    <strong>Comment:</strong>
    <%= comment.body %>
  </p>

  <%= button_to "Destroy comment", [@article, comment], method: :delete %>
</div>

# app/models/article.rb -- only the first string field gets a default
# presence validation; add anything past that yourself.
class Article
  validates :title, presence: true
  validates :body, presence: true, length: { minimum: 10 }
end