I have been developing a new app using a lot of decorators and presenters. I've been highly influenced by Avdi (both his blog and his Objects in Rails book) and Steve Klabnik, but now I have a problem. My decorators are starting to get big. I'll have to use concerns for that. Expect another post about it (maybe).

I want to warn you that I am not by any means an expert on the subject. So I'm more than open to any feedback you can give me.

All presenters are decorators, but not all decorators are presenters.

A decorator is a class that adds some sort of functionality to another class. A presenter is a class that adds some sort of presentation formatting functionality to another class.

Steve Klabnik

I think that what a decorator should do is give a standard API for the processed data stored somewhere (probably a database). I don't think a decorator should be in charge of the markup of the presented data. For me, it makes no sense that the decorator (that is processed data for me), knows how it is being shown to the user.

That said, I think the same decorator should be used to build the JSON response and the HTML response. The HTML markup and JSON structure are just \'markup\'. So, they can be treated as templates.

So, where do the logic in the template goes?

I had this discussion with a friend, and he asked me that, with this approach, how would he add elements to a view depending on a condition, for example, the famous \'You are logged in as nhocki\' if there's a user logged in.

I think this should be created inside helper methods that takes a block. Inside that block you can use the decorator methods to show the needed data. You can use those methods in your JSON builder too (try out the new Jbuilder). You could bundle up these methods and re-use them in your projects too.

Here's an example:

# https://gist.github.com/nhocki/1714913

# app/models/user.rb
# User model has name & username
# Only the username is required (signup with Github for example)
class User < ActiveRecord::Base
  validates :username, presence: true

# app/decorators/user_decorator.rb
# Let's use the Draper gem for the decorators
class UserDecorator < ApplicationDecorator
  decorates :user

  def display_name
    user.name.presence || user.username

  # Formatting the data is OK. I'm against adding the markup
  def salary

# app/views/users/index.html.erb

# current_user is a presenter
<%= logged_in_area do %>
  You are logged in as <%= current_user.display_name %>
<% end %>

# viewing @some_users's profile (which is also a presenter)
<%= admin_area do %>
  <span class="salary"><%= @some_user.salary %></span>
<% end %>

# app/helpers/application_helpers.rb
# We all hate helpers, but this time, they're useful.
def admin_area(&block)
  content_tag(:div, capture(&block), class: 'admin') if current_user.admin?

Maybe I have the names wrong

As I mentioned before, I'm no expert on design patterns. And I'm pretty new to presenters and decorators. So, maybe my approach has a different name. I don't really care how this is called, I feel it's a more natural approach and it keeps the template and the processed data in different parts.

Please provide feedback

It is also possible that I have it all wrong, that's why I'm really interested in your opinion. I'm eager to learn about everything from anyone, so I would love to have some feedback about this. You can post a comment below or send me a message on twitter.