Presenters In Rails

Contain your domain knowledge

Mr Rogers / @rcode5

Why would we need more?

If we try to follow the

Single Responsibility Principle

MVC may not be enough

Feature

            As a visitor to the site
            I can see a list of all users and their ages
            Sorted by name
            So I can easily find out demographics of users in our system
          

Simplistic Rails

# users_controller.rb
def index
  @users = User.all
end

/ users/show.slim
h2 Users
  table
    - @users.select{|u| u.active}.sort_by{|u| [u.first_name, u.last_name].map(&:downcase)}.each do |user|
      tr
        td.name = "#{user.first_name} #{user.last_name}"
        td.age = "#{Time.zone.now.year - user.birthdate.year}"

Display logic is contained in the view

Alternate Plain ol' Rails

# user.rb
class User
  scope :by_name, -> { order('first_name, last_name') }
  scope :active, -> { where(:active => true) }
end

# users_controller.rb
def index
  @users = User.active.by_name
end

/ users/show.slim
h2 Users
  table
    - @users.each do |user|
      tr
        td.name = "#{user.first_name} #{user.last_name}"
        td.age = "#{Time.zone.now.year - user.birthdate.year}"

Less display logic in the view

Rails + a Presenter (or two)

# user.rb
class User
  scope :by_name, -> { order('first_name, last_name') }
  scope :active, -> { where(:active => true) }
end

# users_controller.rb
def index
  @users = UsersPresenter.new
end

/ users/show.slim
h2 Users
  table
    - @users.each do |user|
      tr
        td.name = user.name
        td.age = user.age

The Presenters

# users_presenter.rb
class UsersPresenter
  def users
    @users ||= User.active.by_name.map{|u| UserPresenter.new(u)}
  end
end

# user_presenter.rb
class UserPresenter

  attr_reader :user

  def initialize(user)
    @user = user
  end
  def name
    @name ||= [user.first_name, user.last_name].join(" ")
  end
  def age
    @age ||= (Time.zone.now.year - user.birthdate.year)
  end
end

Display Logic is in the presenter

Thanks for not sleeping

References/Reading list

• The Carbon Emitter - Presenters to Widgets
• Code Climate Blog - 7 Patterns to Refactor Fat ActiveRecord Models
• Jay Fields' Thoughts - Rails Model View Controller + Presenter?
• Jamis Buck - Skinny Controller, Fat Model
• Single Responsibility Principle (Wikipedia)
• Martin Fowler's GUI Patterns
• ABC Metric for Code Complexity

Slides

• http://github.com/bunnymatic/presenters-in-rails-presentation.git

Mr Rogers jon@carbonfive.com bunnymatic @rcode5