Draper: View Models for Rails

Quick Start

1. Add gem 'draper' to your Gemfile and bundle
2. Run rails g draper:model YourModel
3. Edit app/decorators/your_model_decorator.rb using:
4. h to proxy to Rails/application helpers like h.current_user
5. model to access the wrapped object like model.created_at
6. Wrap models in your controller with the decorator using:
7. .find automatic lookup & wrap: ArticleDecorator.find(1)
8. .decorate method with single object or collection: ArticleDecorator.decorate(Article.all)
9. .new method with single object: ArticleDecorator.new(Article.first)
10. Call and output the methods in your view templates as normal

Goals

This gem makes it easy to apply the decorator pattern to the data models in a Rails application. This gives you three wins:

1. Replace most helpers with an object-oriented approach
2. Filter data at the presentation level
3. Enforce an interface between your controllers and view templates.

1. Object Oriented Helpers

Why hate helpers? In Ruby/Rails we approach everything from an Object-Oriented perspective, then with helpers we get procedural.The job of a helper is to take in data and output a presentation-ready string. We can do that with a decorator.

A decorator wraps an object with presentation-related accessor methods. For instance, if you had an Article object, then the decorator could override .published_at to use formatted output like this:

class ArticleDecorator < Draper::Base
  decorates :article
  def published_at
    date = h.content_tag(:span, model.published_at.strftime("%A, %B %e").squeeze(" "), :class => 'date')
    time = h.content_tag(:span, model.published_at.strftime("%l:%M%p"), :class => 'time').delete(" ")
    h.content_tag :span, date + time, :class => 'created_at'
  end
end

2. View-Layer Data Filtering

Have you ever written a to_xml or to_json method in your model? Did it feel weird to put presentation logic in your model?

Or, in the course of formatting this data, did you wish you could access current_user down in the model? Maybe for guests your to_json is only going to show three attributes, but if the user is an admin they get to see them all.

How would you handle this in the model layer? You'd probably pass the current_user or some role/flag down to to_json. That should still feel slimy.

When you use a decorator you have the power of a Ruby object but it's a part of the view layer. This is where your to_xml belongs. You can access your current_user helper method using the h proxy available in the decorator:

class ArticleDecorator < Draper::Base
  decorates :article
  ADMIN_VISIBLE_ATTRIBUTES = [:title, :body, :author, :status]
  PUBLIC_VISIBLE_ATTRIBUTES = [:title, :body]

  def to_xml
    attr_set = h.current_user.admin? ? ADMIN_VISIBLE_ATTRIBUTES : PUBLIC_VISIBLE_ATTRIBUTES
    model.to_xml(:only => attr_set)
  end
end

3. Enforcing an Interface

Want to strictly control what methods are proxied to the original object? Use denies or allows.

Using denies

The denies method takes a blacklist approach. For instance:

class ArticleDecorator < Draper::Base
  decorates :article
  denies :title
end

Then, to test it:

ruby-1.9.2-p290 :001 > ad = ArticleDecorator.find(1)
 => #<ArticleDecorator:0x000001020d7728 @model=#<Article id: 1, title: "Hello, World">> 
ruby-1.9.2-p290 :002 > ad.title
NoMethodError: undefined method `title' for #<ArticleDecorator:0x000001020d7728>

Using allows

A better approach is to define a whitelist using allows:

class ArticleDecorator < Draper::Base
  decorates :article
  allows :title, :description
end

ruby-1.9.2-p290 :001 > ad = ArticleDecorator.find(1)
 => #<ArticleDecorator:0x000001020d7728 @model=#<Article id: 1, title: "Hello, World">> 
ruby-1.9.2-p290 :002 > ad.title
 => "Hello, World"
ruby-1.9.2-p290 :003 > ad.created_at
NoMethodError: undefined method `created_at' for #<ArticleDecorator:0x000001020d7728>

Up and Running

Setup

Add the dependency to your Gemfile:

gem "draper"

Run bundle:

bundle

Disable Rails Helper Generation (Optional)

When you generate a scaffold, Rails will create a matching helper file. If you're using decorators, this is probably unnecessary. You can disable the helper file creation by adding this to your config/application.rb

config.generators do |g|
  g.helper false
end

If you want a helper, you can still call rails generate helper directly.

Generate the Decorator

To decorate a model named Article:

rails generate draper:model Article

Writing Methods

Open the decorator model (ex: app/decorators/article_decorator.rb) and add normal instance methods. To access the wrapped source object, use the model method:

class ArticleDecorator < Draper::Base
  decorates :article
  
  def author_name
    model.author.first_name + " " + model.author.last_name
  end
end

Using Existing Helpers

You probably want to make use of Rails helpers and those defined in your application. Use the helpers or h method proxy:

class ArticleDecorator < Draper::Base
  decorates :article
  
  def published_at
    date = h.content_tag(:span, model.published_at.strftime("%A, %B %e").squeeze(" "), :class => 'date')
    time = h.content_tag(:span, model.published_at.strftime("%l:%M%p"), :class => 'time').delete(" ")
    h.content_tag :span, date + time, :class => 'created_at'
  end
end

In the Controller

When writing your controller actions, you have three options:

• Call .new and pass in the object to be wrapped

  ArticleDecorator.new(Article.find(params[:id]))`
  

• Call .decorate and pass in an object or collection of objects to be wrapped:

  ArticleDecorator.decorate(Article.first) # Returns one instance of ArticleDecorator
  ArticleDecorator.decorate(Article.all)   # Returns an array of ArticleDecorator instances
  

• Call .find to do automatically do a lookup on the decorates class:

  ArticleDecorator.find(1)
  

In Your Views

Use the new methods in your views like any other model method (ex: @article.published_at):

<h1><%= @article.title %> <%= @article.published_at %></h1>

Possible Decoration Methods

Here are some ideas of what you might do in decorator methods:

• Implement output formatting for to_csv, to_json, or to_xml
• Format dates and times using strftime
• Implement a commonly used representation of the data object like a .name method that combines first_name and last_name attributes

Example Using a Decorator

For a brief tutorial with sample project, check this out: http://tutorials.jumpstartlab.com/rails/topics/decorators.html

Say I have a publishing system with Article resources. My designer decides that whenever we print the published_at timestamp, it should be constructed like this:

<span class='published_at'>
  <span class='date'>Monday, May 6</span>
  <span class='time'>8:52AM</span>
</span>

Could we build that using a partial? Yes. A helper? Uh-huh. But the point of the decorator is to encapsulate logic just like we would a method in our models. Here's how to implement it.

First, follow the steps above to add the dependency and update your bundle.

Since we're talking about the Article model we'll create an ArticleDecorator class. You could do it by hand, but use the provided generator:

rails generate draper:model Article

Now open up the created app/decorators/article_decorator.rb and you'll find an ArticleDecorator class. Add this method:

def published_at
  date = h.content_tag(:span, model.published_at.strftime("%A, %B %e").squeeze(" "), :class => 'date')
  time = h.content_tag(:span, model.published_at.strftime("%l:%M%p").delete(" "), :class => 'time')
  h.content_tag :span, date + time, :class => 'published_at'
end

Then you need to perform the wrapping in your controller. Here's the simplest method:

class ArticlesController < ApplicationController
  def show
    @article = ArticleDecorator.find params[:id]
  end
end

Then within your views you can utilize both the normal data methods and your new presentation methods:

<%= @article.published_at %>

Ta-da! Object-oriented data formatting for your view layer. Below is the complete decorator with extra comments removed:

class ArticleDecorator < Draper::Base
  decorates :article
  
  def published_at
    date = h.content_tag(:span, model.published_at.strftime("%A, %B %e").squeeze(" "), :class => 'date')
    time = h.content_tag(:span, model.published_at.strftime("%l:%M%p"), :class => 'time').delete(" ")
    h.content_tag :span, date + time, :class => 'published_at'
  end
end

Issues / Pending

• Documentation
  • Keep revising Readme for better organization/clarity
  • Make clear the pattern of overriding accessor methods of the wrapped model
  • Build sample Rails application
  • Document the lazy_helpers feature
• Generators
  • Test coverage for generators (help!)
  • Implement hook so generating a controller/scaffold generates a decorator
  • Change generator to create an ApplicationDecorator, then inherit from that
• Other
  • Implement a HATEOAS helper, maybe as a separate gem
  • Build a fly website like http://fabricationgem.com

License

(The MIT License)

Copyright © 2011 Jeff Casimir

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the ‘Software’), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED ‘AS IS’, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.