Draper: View Models for Rails
Quick Start
- Add
gem 'draper'
to yourGemfile
andbundle
- Run
rails g draper:decorator YourModel
- Edit
app/decorators/[your_model]_decorator.rb
using: h
to proxy to Rails/application helpers likeh.current_user
model
to access the wrapped object likemodel.created_at
- Put common decorations in
app/decorators/application.rb
- Wrap models in your controller with the decorator using:
.find
automatic lookup & wrap ex:ArticleDecorator.find(1)
.decorate
method with single object or collection, ex:ArticleDecorator.decorate(Article.all)
.new
method with single object ex:ArticleDecorator.new(Article.first)
- Output the instance methods in your view templates
ex:
@article_decorator.created_at
Goals
This gem makes it easy to apply the decorator pattern to domain models in a Rails application. This pattern gives you three wins:
- Replace most helpers with an object-oriented approach
- Filter data at the presentation level
- Enforce an interface between your controllers and view templates.
1. Object Oriented Helpers
Why hate normal 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 < ApplicationDecorator
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_json
belongs. You can access your current_user
helper method using the h
proxy available in the decorator:
class ArticleDecorator < ApplicationDecorator
decorates :article
ADMIN_VISIBLE_ATTRIBUTES = [:title, :body, :author, :status]
PUBLIC_VISIBLE_ATTRIBUTES = [:title, :body]
def to_json
attr_set = h.current_user.admin? ? ADMIN_VISIBLE_ATTRIBUTES : PUBLIC_VISIBLE_ATTRIBUTES
model.to_json(: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
.
denies
Using The denies
method takes a blacklist approach. For instance:
class ArticleDecorator < ApplicationDecorator
decorates :article
denies :title
end
Then, to test it:
> ad = ArticleDecorator.find(1)
=> #<ArticleDecorator:0x000001020d7728 @model=#<Article id: 1, title: "Hello, World">>
> ad.title
NoMethodError: undefined method `title' for #<ArticleDecorator:0x000001020d7728>
allows
Using A better approach is to define a whitelist using allows
:
class ArticleDecorator < ApplicationDecorator
decorates :article
allows :title, :description
end
> ad = ArticleDecorator.find(1)
=> #<ArticleDecorator:0x000001020d7728 @model=#<Article id: 1, title: "Hello, World">>
> ad.title
=> "Hello, World"
> 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.
Add DecoratorGenerator to ActiveRecord Generator (Optional)
Add the following to your config/application.rb
config.generators do |g|
g.orm :decorator, :invoke_after_finished => "active_record:model"
end
From now on, every model you generate will first invoke the DecoratorGenerator. The Decorator will then invoke the active_record:model Generator.
Generate the Decorator
To decorate a model named Article
:
rails generate draper:decorator 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 < ApplicationDecorator
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 < ApplicationDecorator
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
Lazy Helpers
Hate seeing that h.
proxy all over? Willing to mix a bazillion methods into your decorator? Then try lazy helpers:
class ArticleDecorator < ApplicationDecorator
decorates :article
lazy_helpers
def published_at
date = content_tag(:span, model.published_at.strftime("%A, %B %e").squeeze(" "), :class => 'date')
time = content_tag(:span, model.published_at.strftime("%l:%M%p"), :class => 'time').delete(" ")
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 thedecorates
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
, orto_xml
- Format dates and times using
strftime
- Implement a commonly used representation of the data object like a
.name
method that combinesfirst_name
andlast_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:decorator 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 < ApplicationDecorator
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
- Add more information about using "context"
- Add information about the
.decorator
method - Make clear the pattern of overriding accessor methods of the wrapped model
- Build sample Rails application(s)
- Add a short screencast
- Add YARD documentation to source
- Add a section about contributing
- Generators
- Implement hook so generating a controller/scaffold generates a decorator
- Add generators for...
draper:model
: Model + Decoratordraper:controller
: Controller setup with decoration callsdraper:scaffold
: Controller, Model, Decorator, Views, Tests
- 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.