Using Decorators in Rails

Have you ever opened a view in your Ruby on Rails app and seen something like this:

# users/show.html.erb
<% if user.avatar.present? %>
  <%= image_tag(user.avatar) %>
<% else %>
  <div><%= user.fullname %></div>
<% end %>

This is a somewhat common code smell, and should be avoided.

What’s wrong with if/else in a view?!

It adds extra cognitive processing while considering the design of a page.

Instead of “Ok, when no avatar is present, it will always fall back to the users name in a div”, the design choice becomes “It could fallback, but that might not always be the case”.

Enter Decorators

Decorators (I’m fond of Draper) take that presentational logic and puts it somewhere out of the way.

So the above code can become:

# app/decorators/user_decorator.rb
class UserDecorator < Draper::Decorator
  delegate_all

  def branding
   if object.avatar.present?
     h.image_tag(object.avatar)
   else
     h.content_tag(:div, user.fullname)
   end
  end
end

# users/show.html.erb
<%= user.branding %>

Expanding it further

The above example will get you pretty far, but you can reduce the amount of logic you’re working with further.

ApplicationDecorator

I’ve found adding a parent class containing commonly used methods can reduce the amount of duplication in decorators, e.g.:

# app/decorators/application_decorator.rb
class ApplicationDecorator < Draper::Decorator

  private
  # Call cms_content(object.some_field) to have a consistent CMS output.
  def cms_content(body)
    h.sanitize(body, tags: %w(p a ul ol li), attributes: %w(href))
  end
end

Contextual Decorators

In cases (such as admin panels), you might want to have different decoration while still inheriting from the parent decorators, e.g.:

# app/decorators/admin/user_decorator.rb
class Admin::UserDecorator < UserDecorator
  # Tell the Draper we're decorating the User model.
  decorates User
  delegate_all

  # Fallback to gavatar image in the admin panels
  def avatar
    if object.avatar.present?
      h.image_tag(object.avatar)
    else
      h.image_tag(object.gavatar_url)
    end
  end
end