article.rb

class Article < ActiveRecord::Base
  validates :title, :category, presence: true
  belongs_to :category
end

category.rb

class Category < ActiveRecord::Base
  validates :name, presence: true
  has_many :articles
end

index.html.slim

h1 Artigos

table.table.table-striped
  thead
    tr
      th Título
      th Publicado em
      th Categoria
      th

  tbody
    - @articles.order(published_at: :desc).order(:title).each do |article|
      tr
        td = article.title
        td = l article.published_at
        td = article.category.name
        td
          .btn-group.pull-right
            = link_to 'Ver', article, class: 'btn btn-default btn-sm'
            = link_to 'Editar', edit_article_path(article), class: 'btn btn-default btn-sm'
            = link_to 'Excluir', article, :confirm => 'Tem certeza?', :method => :delete, class: 'btn btn-default btn-sm'

br

= link_to 'Criar Artigo', new_article_path, class: 'btn btn-primary'

show.html.slim

- if notice
  .alert.alert-info.alert-dismissable
    button type="button" class="close" data-dismiss="alert" aria-hidden="true" × 
    = notice

h1 
  = @article.title
  .pull-right
    = link_to 'Editar', edit_article_path(@article), class: 'btn btn-default'

hr

strong = @article.category.name

p = l @article.published_at
= simple_format @article.body

terminal

rails g migration add_category_id_to_articles category_id:integer:index
rake db:migrate