“Object on Rails” example refactored (models only)

post.rb

class Post < ActiveRecord::Base
  include Taggable

  validates :title, presence: true

  before_create :set_defaults

  scope :most_recent, ->{ order('pubdate DESC').limit(10) }

  def picture?
    image_url.present?
  end

  def prev
    Post.order('pubdate DESC').where('pubdate < ?', pubdate).first
  end

  def next
    Post.order('pubdate ASC').where('pubdate > ?', pubdate).first
  end

  private

  def set_defaults
    self.pubdate ||= Time.now
    self.body    ||= 'Nothing to see here'
  end
end

tag.rb

class Tag < ActiveRecord::Base
  scope :alphabetical, ->{ order(:name) }
end

taggable.rb

module Taggable
  extend ActiveSupport::Concern

  included do
    has_many :taggings
    has_many :tags, through: :taggings

    scope :tagged_with, ->(tag_name) do
      joins(:tags).where(tags: { name: tag_name }) 
    end
  end

  def tag_list
    tags.pluck(:name).join(', ')
  end

  def tag_list=(tag_list)
    assign_tag_list tag_list
  end

  private

  def assign_tag_list(tag_list)
    tag_names = tag_list.split(',').map(&:strip).uniq

    self.tags = tag_names.map do |tag_name| 
      Tag.where(name: tag_name).first_or_initialize
    end
  end
end

tagging.rb

class Tagging < ActiveRecord::Base
  belongs_to :article
  belongs_to :tag
end