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Decorators/View-Models for Rails Applications

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Readme.markdown

Heads Up!

This gem is not yet production ready. The API is sure to change quickly and there could be side-effects I haven't discovered yet.

Please only use it if you're playing around and helping me experiment! Thanks :)

Draper

This gem makes it easy to apply the decorator pattern to the 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 job in an OO style with a decorator.

In general, a decorator wraps an object with presentation-related accessor methods. For instance, if you had an Article object, then a decorator might add instance methods like .formatted_published_at or .formatted_title that output actual HTML.

For example:

class ArticleDecorator < Draper::Base
  def formatted_published_at
    date = content_tag(:span, published_at.strftime("%A, %B %e").squeeze(" "), :class => 'date')
    time = content_tag(:span, published_at.strftime("%l:%M%p"), :class => 'time').delete(" ")
    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 what is essentially view 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. It has access to the core data from the model, but it also knows about current_user because it can see the ApplicationHelper where that method is typically defined.

For example:

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

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

How is it implemented?

To implement the pattern in Rails we can:

  1. Write a wrapper class with the decoration methods
  2. Wrap the data object
  3. Utilize those methods within our view layer

How do you utilize this gem in your application?

Here are the steps to utilizing this gem:

Add the dependency to your Gemfile:

gem "draper"

Run bundle:

bundle

Create a decorator for your model (ex: Article)

rails generate draper:model Article

Open the decorator model (ex: app/decorators/article_decorator.rb)

Add your new formatting methods as normal instance or class methods. You have access to the Rails helpers from the following classes:

ActionView::Helpers::TagHelper
ActionView::Helpers::UrlHelper
ActionView::Helpers::TextHelper

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

An Interface with Allows/Denies

A proper interface defines a contract between two objects. One purpose of the decorator pattern is to define an interface between your data model and the view template.

You are provided class methods allows and denies to control exactly which of the subject's methods are available. By default, all of the subject's methods can be accessed.

For example, say you want to prevent access to the :title method. You'd use denies like this:

  class ArticleDecorator < Draper::Base
    denies :title
  end

denies uses a blacklist approach. Note that, as of the current version, denying :title does not affect related methods like :title=, :title?, etc.

A better idea is a whitelist approach using allows:

  class ArticleDecorator < Draper::Base
    allows :title, :body, :author
  end

Now only those methods and any defined in the decorator class itself can be accessed directly.

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

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, update your bundle, then run the rails generate decorator:setup to prepare your app.

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

ASIDE: Unfortunately, due to the current implementation of content_tag, you can't use the style of sending the content is as a block or you'll get an error about undefined method 'output_buffer='. Passing in the content as the second argument, as above, works fine.

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

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

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

<%= @article.formatted_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
  def formatted_published_at
    date = content_tag(:span, published_at.strftime("%A, %B %e").squeeze(" "), :class => 'date')
    time = content_tag(:span, published_at.strftime("%l:%M%p"), :class => 'time').delete(" ")
    content_tag :span, date + time, :class => 'created_at'
  end
end

Issues / Pending

  • Test coverage for generators
  • Ability to decorate multiple objects at once, ex: ArticleDecorator.decorate(Article.all)
  • Revise readme to better explain interface pattern
  • Build sample Rails application
  • Consider: ArticleDecorator.new(1) does the equivalent of ArticleDecorator.new(Article.find(1))

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.

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