Draper: View Models for Rails
Draper adds an object-oriented layer of presentation logic to your Rails application.
Without Draper, this functionality might have been tangled up in procedural helpers or adding bulk to your models. With Draper decorators, you can wrap your models with presentation-related logic to organise - and test - this layer of your app much more effectively.
Why Use a Decorator?
Imagine your application has an Article
model. With Draper, you’d create a corresponding ArticleDecorator
. The decorator wraps the model, and deals only with presentational concerns. In the controller, you decorate the article before handing it off to the view:
“by
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. But whenever you start needing logic in the view or start thinking about a helper method, you can implement a method on the decorator instead.
Let’s look at how you could convert an existing Rails helper to a decorator method. You have this existing helper:
“by
app/helpers/articles_helper.rb
def publication_status(article) if article.published? “Published at #%B %e’)” else “Unpublished” end end
“
But it makes you a little uncomfortable. publication_status
lives in a nebulous namespace spread across all controllers and view. Down the road, you might want to display the publication status of a Book
. And, of course, your design calls for a slighly different formatting to the date for a Book
.
Now your helper method can either switch based on the input class type (poor Ruby style), or you break it out into two methods, book_publication_status
and article_publication_status
. And keep adding methods for each publication type…to the global helper namespace. And you’ll have to remember all the names. Ick.
Ruby thrives when we use Object-Oriented style. If you didn’t know Rails’ helpers existed, you’d probably imagine that your view template could feature something like this:
“b <%= @article.publication_status %>
“
Without a decorator, you’d have to implement the publication_status
method in the Article
model. That method is presentation-centric, and thus does not belong in a model.
Instead, you implement a decorator:
“by
app/decorators/article_decorator.rb
class ArticleDecorator < Draper::Decorator delegate_all
def publication_status if published? “Published at #published_at” else “Unpublished” end end
def published_at object.published_at.strftime(“%A, %B %e”) end end
“
Within the publication_status
method we use the published?
method. Where does that come from? It’s a method of the source Article
, whose methods have been made available on the decorator by the delegate_all
call above.
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 complex data for user display * 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:
“by gem ‘draper’, ‘~> 1.3’
“
And run bundle install
within your app’s directory.
If you’re upgrading from a 0.x release, the major changes are outlined in the wiki.
Writing Decorators
Decorators inherit from Draper::Decorator
, live in your app/decorators
directory, and are named for the model that they decorate:
“by
app/decorators/article_decorator.rb
class ArticleDecorator < Draper::Decorator
…
end
“
Generators
When you have Draper installed and generate a controller…
“ails generate resource Article
“
…you’ll get a decorator for free!
But if the Article
model already exists, you can run…
“ails generate decorator Article
“
…to create the ArticleDecorator
.
Accessing Helpers
Normal Rails helpers are still useful for lots of tasks. Both Rails’ provided helpers and those defined in your app can be accessed within a decorator via the h
method:
“by class ArticleDecorator < Draper::Decorator def emphatic h.content_tag(:strong, “Awesome”) end end
“
If writing h.
frequently is getting you down, you can add…
“nclude Draper::LazyHelpers
“
…at the top of your decorator class - you’ll mix in a bazillion methods and never have to type h.
again.
(Note: the capture
method is only available through h
or helpers
)
Accessing the model
When writing decorator methods you’ll usually need to access the wrapped model. While you may choose to use delegation (covered below) for convenience, you can always use the object
(or its alias model
):
“by class ArticleDecorator < Draper::Decorator def published_at object.published_at.strftime(“%A, %B %e”) end end
“
Decorating Objects
Single Objects
Ok, so you’ve written a sweet decorator, now you’re going to want to put it into action! A simple option is to call the decorate
method on your model:
“by @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:
“by @widget = ProductDecorator.new(Widget.first)
or, equivalently
@widget = ProductDecorator.decorate(Widget.first)
“
Collections
Decorating Individual Elements
If you have a collection of objects, you can decorate them all in one fell swoop:
“by @articles = ArticleDecorator.decorate_collection(Article.all)
“
If your collection is an ActiveRecord query, you can use this:
“by @articles = Article.popular.decorate
“
Note: In Rails 3, the .all
method returns an array and not a query. Thus you cannot use the technique of Article.all.decorate
in Rails 3. In Rails 4, .all
returns a query so this techique would work fine.
Decorating the Collection Itself
If you want to add methods to your decorated collection (for example, for pagination), you can subclass Draper::CollectionDecorator
:
“by
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 decorates each item by calling the decorate
method. Alternatively, you can specify a decorator by overriding the collection decorator’s decorator_class
method, or by passing the :with
option to the constructor.
Using pagination
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 delegate to the object
:
“by class PaginatingDecorator < Draper::CollectionDecorator delegate :current_page, :total_pages, :limit_value, :entry_name, :total_count, :offset_value, :last_page? end
“
The delegate
method used here is the same as that added by Active Support, except that the :to
option is not required; it defaults to :object
when omitted.
will_paginate needs the following delegations:
“by delegate :current_page, :per_page, :offset, :total_entries, :total_pages
“
Decorating Associated Objects
You can automatically decorate associated models when the primary model is decorated. Assuming an Article
model has an associated Author
object:
“by class ArticleDecorator < Draper::Decorator decorates_association :author end
“
When ArticleDecorator
decorates an Article
, it will also use AuthorDecorator
to decorate the associated Author
.
Decorated Finders
You can call decorates_finders
in a decorator…
“by class ArticleDecorator < Draper::Decorator decorates_finders end
“
…which allows you to then call all the normal ActiveRecord-style finders on your ArticleDecorator
and they’ll return decorated objects:
“by @article = ArticleDecorator.find(params[:id])
“
When to Decorate Objects
Decorators are supposed to behave very much like the models they decorate, and for that reason it is very tempting to just decorate your objects at the start of your controller action and then use the decorators throughout. Don’t.
Because decorators are designed to be consumed by the view, you should only be accessing them there. Manipulate your models to get things ready, then decorate at the last minute, right before you render the view. This avoids many of the common pitfalls that arise from attempting to modify decorators (in particular, collection decorators) after creating them.
To help you make your decorators read-only, we have the decorates_assigned
method in your controller. It adds a helper method that returns the decorated version of an instance variable:
“by
app/controllers/articles_controller.rb
class ArticlesController < ApplicationController decorates_assigned :article
def show @article = Article.find(params[:id]) end end
“
The decorates_assigned :article
bit is roughly equivalent to
“by def article @decorated_article ||= @article.decorate end helper_method :article
“
This means that you can just replace @article
with article
in your views and you’ll have access to an ArticleDecorator object instead. In your controller you can continue to use the @article
instance variable to manipulate the model - for example, @article.comments.build
to add a new blank comment for a form.
Testing
Draper supports RSpec, MiniTest::Rails, and Test::Unit, and will add the appropriate tests when you generate a decorator.
RSpec
Your specs are expected to live in spec/decorators
. If you use a different path, you need to tag them with type: :decorator
.
In a controller spec, you might want to check whether your instance variables are being decorated properly. You can use the handy predicate matchers:
“by 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
In your Spork.prefork
block of spec_helper.rb
, add this:
“by require ‘draper/test/rspec_integration’
“
Isolated Tests
In tests, Draper needs to build a view context to access helper methods. By default, it will create an ApplicationController
and then use its view context. If you are speeding up your test suite by testing each component in isolation, you can eliminate this dependency by putting the following in your spec_helper
or similar:
“by Draper::ViewContext.test_strategy :fast
“
In doing so, your decorators will no longer have access to your application’s helpers. If you need to selectively include such helpers, you can pass a block:
“by Draper::ViewContext.test_strategy :fast do include ApplicationHelper end
“
Stubbing Route Helper Functions
If you are writing isolated tests for Draper methods that call route helper methods, you can stub them instead of needing to require Rails.
If you are using RSpec, minitest-rails, or the Test::Unit syntax of minitest, you already have access to the Draper helpers
in your tests since they inherit from Draper::TestCase
. If you are using minitest’s spec syntax without minitest-rails, you can explicitly include the Draper helpers
:
“by describe YourDecorator do include Draper::ViewHelpers end
“
Then you can stub the specific route helper functions you need using your preferred stubbing technique (this example uses RSpec’s stub
method):
“by helpers.stub(users_path: ‘/users’)
“
Advanced usage
Shared Decorator Methods
You might have several decorators that share similar needs. Since decorators are just Ruby objects, you can use any normal Ruby technique for sharing functionality.
In Rails controllers, common functionality is organized by having all controllers inherit from ApplicationController
. You can apply this same pattern to your decorators:
“by
app/decorators/application_decorator.rb
class ApplicationDecorator < Draper::Decorator
…
end
“
Then modify your decorators to inherit from that ApplicationDecorator
instead of directly from Draper::Decorator
:
“by class ArticleDecorator < ApplicationDecorator # decorator methods end
“
Delegating Methods
When your decorator calls delegate_all
, any method called on the decorator not defined in the decorator itself will be delegated to the decorated object. This is a very permissive interface.
If you want to strictly control which methods are called within views, you can choose to only delegate certain methods from the decorator to the source model:
“by class ArticleDecorator < Draper::Decorator delegate :title, :body end
“
We omit the :to
argument here as it defaults to the object
being decorated. You could choose to delegate methods to other places like this:
“by class ArticleDecorator < Draper::Decorator delegate :title, :body delegate :name, :title, to: :author, prefix: true end
“
From your view template, assuming @article
is decorated, you could do any of the following:
“by @article.title # Returns the article’s .title
@article.body # Returns the article’s .body
@article.author_name # Returns the article’s author.name
@article.author_title # Returns the article’s author.title
“
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:
“by Article.first.decorate(context: :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:
“by class ArticleDecorator < Draper::Decorator decorates_association :author, context: “bar” end
“
or, if you want to modify the parent’s context, use a lambda that takes a hash and returns a new hash:
“by 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(s) to perform the decoration. If you want use a specific decorator, you can use the :with
option:
“by 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:
“by class ArticleDecorator < Draper::Decorator decorates_association :comments, scope: :recent end
“
Proxying Class Methods
If you want to proxy class methods to the wrapped model class, 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:
“by class MySpecialArticleDecorator < Draper::Decorator decorates :article end
“
This is only necessary when proxying 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 (including versions of Mongoid prior to 3.0), 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 ([email protected])
- Steve Klabnik ([email protected])
- Vasiliy Ermolovich
- Andrew Haines