Draper: View Models for Rails

Draper adds a nicely-separated object-oriented layer of presentation logic to your Rails apps. Previously, this logic might have been tangled up in procedural helpers, or contributing to your fat models' weight problems. Now, you can wrap your models up in decorators to organise - and test - this layer of your app much more effectively.

Overview

With Draper, your Article model has a corresponding ArticleDecorator. The decorator wraps the model, and deals only with presentational concerns. In the controller, you simply decorate your article before handing it off to the view.

# app/controllers/articles_controller.rb
def show
  @article = Article.find(params[:id]).decorate
end

In the view, you can use the decorator in exactly the same way as you would have used the model. The difference is, any time you find yourself needing to write a helper, you can implement a method on the decorator instead. For example, this helper:

# app/helpers/articles_helper.rb
def publication_status(article)
  if article.published?
    "Published at #{article.published_at.strftime('%A, %B %e')}"
  else
    "Unpublished"
  end
end

could be better written as:

# app/decorators/article_decorator.rb
class ArticleDecorator < Draper::Decorator
  def publication_status
    if published?
      "Published at #{published_at}"
    else
      "Unpublished"
    end
  end

  def published_at
    source.published_at.strftime("%A, %B %e")
  end
end

Notice that the published? method can be called even though ArticleDecorator doesn't define it - the decorator delegates methods to the source model. However, we can override methods like published_at to add presentation-specific formatting, in which case we access the underlying model using the source method.

You might have heard this sort of decorator called a "presenter", an "exhibit", a "view model", or even just a "view" (in that nomenclature, what Rails calls "views" are actually "templates"). Whatever you call it, it's a great way to replace procedural helpers like the one above with "real" object-oriented programming.

Decorators are the ideal place to:

• format dates and times using strftime,
• define commonly-used representations of an object, like a name method that combines first_name and last_name attributes,
• mark up attributes with a little semantic HTML, like turning a url field into a hyperlink.

Installation

Add Draper to your Gemfile:

gem 'draper', '~> 1.0.0.beta6'

And run bundle install within your app's directory.

Writing decorators

Decorators inherit from Draper::Decorator, live in your app/decorators directory, and are named for the model that they decorate:

# app/decorators/article_decorator.rb
class ArticleDecorator < Draper::Decorator
# ...
end

Generators

When you generate a resource with rails generate resource Article, you get a decorator for free! But if the Article model already exists, you can run rails generate decorator Article to create the ArticleDecorator.

Accessing helpers

Procedural helpers are still useful for generic tasks like generating HTML, and as such you can access all this goodness (both built-in Rails helpers, and your own) through the helpers method:

class ArticleDecorator < Draper::Decorator
  def emphatic
    helpers.content_tag(:strong, "Awesome")
  end
end

To save your typing fingers it's aliased to h. If that's still too much effort, just pop include Draper::LazyHelpers at the top of your decorator class - you'll mix in a bazillion methods and never have to type h. again... if that's your sort of thing.

Accessing the model

Decorators will delegate methods to the model where possible, which means in most cases you can replace a model with a decorator and your view won't notice the difference. When you need to get your hands on the underlying model the source method is your friend (and its aliases model and to_source):

class ArticleDecorator < Draper::Decorator
  def published_at
    source.published_at.strftime("%A, %B %e")
  end
end

Decorating

Single objects

Ok, so you've written a sweet decorator, now you're going to want to put it in action! A simple option is to call the decorate method on your model:

@article = Article.first.decorate

This infers the decorator from the object being decorated. If you want more control - say you want to decorate a Widget with a more general ProductDecorator - then you can instantiate a decorator directly:

@widget = ProductDecorator.new(Widget.first)
# or, equivalently
@widget = ProductDecorator.decorate(Widget.first)

Collections

If you have a whole bunch of objects, you can decorate them all in one fell swoop:

@articles = ArticleDecorator.decorate_collection(Article.all)
# or, for scopes (but not `all`)
@articles = Article.popular.decorate

If you want to add methods to your decorated collection (for example, for pagination), you can subclass Draper::CollectionDecorator:

# app/decorators/articles_decorator.rb
class ArticlesDecorator < Draper::CollectionDecorator
  def page_number
    42
  end
end

# elsewhere...
@articles = ArticlesDecorator.new(Article.all)
# or, equivalently
@articles = ArticlesDecorator.decorate(Article.all)

Draper guesses the decorator used for each item from the name of the collection decorator (ArticlesDecorator becomes ArticleDecorator). If that fails, it falls back to using each item's decorate method. Alternatively, you can specify a decorator by overriding the collection decorator's decorator_class method.

Some pagination gems add methods to ActiveRecord::Relation. For example, Kaminari's paginate helper method requires the collection to implement current_page, total_pages, and limit_value. To expose these on a collection decorator, you can simply delegate to source:

class PaginatingDecorator < Draper::CollectionDecorator
  delegate :current_page, :total_pages, :limit_value, to: :source
end

Handy shortcuts

You can automatically decorate associated models:

class ArticleDecorator < Draper::Decorator
  decorates_association :author
end

And, if you want, you can add decorated finder methods:

class ArticleDecorator < Draper::Decorator
  decorates_finders
end

so that you can do:

@article = ArticleDecorator.find(params[:id])

Testing

Draper supports RSpec and Minitest::Rails out of the box, and should work with Test::Unit as well.

RSpec

Your specs should live in spec/decorators (if not, you need to tag them with type: :decorator).

In controller specs, you might want to check whether your instance variables are being decorated properly. You can use the handy predicate matchers:

assigns(:article).should be_decorated
# or, if you want to be more specific
assigns(:article).should be_decorated_with ArticleDecorator

Note that model.decorate == model, so your existing specs shouldn't break when you add the decoration.

Spork users should require 'draper/test/rspec_integration' in the Spork.prefork block.

Advanced usage

ApplicationDecorator

If you need common methods in your decorators, you can create an ApplicationDecorator:

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

and inherit from it instead of directly from Draper::Decorator.

Enforcing an interface between controllers and views

If you want to strictly control which methods are called in your views, you can restrict the methods that the decorator delegates to the model. Use denies to blacklist methods:

class ArticleDecorator < Draper::Decorator
  # allow everything except `title` and `author` to be delegated
  denies :title, :author
end

or, better, use allows for a whitelist:

class ArticleDecorator < Draper::Decorator
  # only allow `title` and `author` to be delegated to the model
  allows :title, :author
end

You can prevent method delegation altogether using denies_all.

Adding context

If you need to pass extra data to your decorators, you can use a context hash. Methods that create decorators take it as an option, for example

Article.first.decorate(context: {role: :admin})

The value passed to the :context option is then available in the decorator through the context method.

If you use decorates_association, the context of the parent decorator is passed to the associated decorators. You can override this with the :context option:

class ArticleDecorator < Draper::Decorator
  decorates_association :author, context: {foo: "bar"}
end

or, if you simply want to modify the parent's context, use a lambda that takes a hash and returns a new hash:

class ArticleDecorator < Draper::Decorator
  decorates_association :author,
    context: ->(parent_context){ parent_context.merge(foo: "bar") }
end

Specifying decorators

When you're using decorates_association, Draper uses the decorate method on the associated record (or each associated record, in the case of a collection association) to perform the decoration. If you want use a specific decorator, you can use the :with option:

class ArticleDecorator < Draper::Decorator
  decorates_association :author, with: FancyPersonDecorator
end

For a collection association, you can specify a CollectionDecorator subclass, which is applied to the whole collection, or a singular Decorator subclass, which is applied to each item individually.

Scoping associations

If you want your decorated association to be ordered, limited, or otherwise scoped, you can pass a :scope option to decorates_association, which will be applied to the collection before decoration:

class ArticleDecorator < Draper::Decorator
  decorates_association :comments, scope: :recent
end

Breaking with convention

If, as well as instance methods, you want to proxy class methods to the model through the decorator (including when using decorates_finders), Draper needs to know the model class. By default, it assumes that your decorators are named SomeModelDecorator, and then attempts to proxy unknown class methods to SomeModel. If your model name can't be inferred from your decorator name in this way, you need to use the decorates method:

class MySpecialArticleDecorator < Draper::Decorator
  decorates :article
end

You don't need to worry about this if you don't want to proxy class methods.

Making models decoratable

Models get their decorate method from the Draper::Decoratable module, which is included in ActiveRecord::Base and Mongoid::Document by default. If you're using another ORM, or want to decorate plain old Ruby objects, you can include this module manually.

Contributors

Draper was conceived by Jeff Casimir and heavily refined by Steve Klabnik and a great community of open source contributors.

Core Team

• Jeff Casimir (jeff@jumpstartlab.com)
• Steve Klabnik (steve@jumpstartlab.com)
• Vasiliy Ermolovich
• Andrew Haines