# Draper: View Models for Rails

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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:

```ruby
# 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:

```ruby
# 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    "Unpublished"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
```

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 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:

```erb
<%= @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:

```ruby
# 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
    source.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:

```ruby
gem 'draper', '~> 1.0'
```

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](https://github.com/drapergem/draper/wiki/Upgrading-to-1.0).

## Writing Decorators

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

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

### Generators

When you have Draper installed and generate a resource with...

```
rails generate resource Article
```
...you'll 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

Normal Rails helpers are still useful for lots of tasks. Both Rails' provided helper and those defined in your app can be accessed via the `h` method:

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

If writing `h.` frequently is getting you down, you can add...

```
include 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](#delegating-methods)) for convenience, you can always use the `source` (or its alias `model`):

```ruby
class ArticleDecorator < Draper::Decorator
  def published_at
    source.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 in action! A simple option is to call the `decorate` method on your model:

```ruby
@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:

```ruby
@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:

```ruby
@articles = ArticleDecorator.decorate_collection(Article.all)
```

If your collection is an ActiveRecord query, you can use this:

```ruby
@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.

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

```ruby
# 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 using its `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](https://github.com/amatsuda/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 `source`:

```ruby
class PaginatingDecorator < Draper::CollectionDecorator
  delegate :current_page, :total_pages, :limit_value
end
```

The `delegate` method used here is the same as that added by [Active Support](http://api.rubyonrails.org/classes/Module.html#method-i-delegate), except that the `:to` option is not required; it defaults to `:source` when omitted.

[will_paginate](https://github.com/mislav/will_paginate) needs you to `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:

```ruby
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...

```ruby
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:

```ruby
@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:

```ruby
# 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

```ruby
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:

```ruby
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:

```ruby
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:

```ruby
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:

```ruby
Draper::ViewContext.test_strategy :fast do
  include ApplicationHelper
end
```

## 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:

```ruby
# 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`:

```ruby
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 source. 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:

```ruby
class ArticleDecorator < Draper::Decorator
  delegate :title, :body
end
```

We omit the `:to` argument here as it defaults to the `source` object. You could choose to delegate methods to other places like this:

```ruby
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:

```ruby
@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:

```ruby
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:

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

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

```ruby
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:

```ruby
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:

```ruby
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:

```ruby
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](https://github.com/drapergem/draper/wiki/Using-other-ORMs) (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](https://github.com/drapergem/draper/contributors).

### Core Team

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