Lets a Controller exposes a Context object to the View as the single connection point between the two
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Allows the Controller to exposes a Context object to the View. This Context object alone contains all the information passed to the View from the controller.

No more pollution of the View with content helper methods or even worse, instance variables.

The Context object will by default be an instance of ContextExposer::ViewContext, but you can subclass this baseclass to add you own logic for more complex scenarios. This also allows for a more modular approach, where you can easily share or subclass logic between different view contexts. Nice!

The gem comes with integrations ready for easy migration or symbiosis with existing strategies (and gems), such as:

  • exposing of instance variables (Rails default strategy)
  • decent_exposure gem (expose methods)
  • decorates_before_rendering gem (expose decorated instance vars)
  • draper

For more on integration (and migration path) see below ;)


Add this line to your application's Gemfile:

gem 'context_exposer'

And then execute:

$ bundle

Or install it yourself as:

$ gem install context_exposer


Use the exposed method which takes a name of the method to be created on the ViewContext and a block with the logic.


class ApplicationController
  include ContextExposer::BaseController


class PostsController < ApplicationController    
  exposed(:post)  { Post.find params[:id] }
  exposed(:posts) { Post.find params[:id] }

Note: In this example we included the ContextExposer::BaseController in the ApplicationController so that the functionality becomes available for all controllers based on the ApplicationController.

We used the macro turn_off_view_assigns (part of ContextExposer::BaseController) in order to turn off the ability for controllers to pass instance variables to the views.

The view will have the methods post and posts exposed and available on the ctx or view_ctx object.

HAML view example

%h1 Posts
= ctx.posts.each do |post|
  %h2 = post.name

You can also have the exposed methods automatically cache the result in an instance variable, by using the expose_cached variant.

class PostsController < ApplicationController
  include ContextExposer::BaseController

  expose_cached(:post)  { Post.find params[:id] }
  expose_cached(:posts) { Post.find params[:id] }

This is especially useful if used to decorate before rendering, fx by using the gem decorates_before_rendering.


You can also choose to use the class macros made available on ContextExposer::BaseController as Rails loads.

Use :base or :resource or your custom extension to include the ContextExposer controller module of your choice. The macro context_exposer :base is equivalent to writing include ContextExposer::BaseController

class PostsController < ApplicationController
  context_exposer :base

Sublclassing and customizing the ViewContext

You can also define your own subclass of ViewContext and designate an instance of this custom class as your "exposed" target, via view_ctx_classmethod. You can also override the class method of the same name for custom class name construction behavior ;)


class PostsController < ApplicationController
  include ContextExposer::BaseController

  view_ctx_class :posts_view_context

  # One model instance
  exposed(:post)        { Post.find params[:id] } 

  # Relation (for further scoping or lazy load)
  exposed(:posts)       { Post.all } 

  # Array of model instances
  exposed(:post_list)   { Post.all.to_a } 
class PostsViewContext < ContextExposer::ViewContext
  def initialize controller

  def total

  def admin_posts
    return [] unless admin?
    posts.select {|post| post.admin? }


  def current_user

  def admin?

HAML view example

%h1 Admin Posts
= ctx.admin_posts.each do |post|
  %h2 = post.name

This opens up some amazing possibilities to really put the logic where it belongs.The custom ViewContext would benefit from having the "admin" and "user" logic extracted either to separate modules or a custom ViewContext base class ;)

This approach opens up many new exciting ways to slice and dice your logic in a much better way, a new MVC-C architecture, the extra "C" for Context.


The ResourceController automatically sets up the typical singular and plural-form resource helpers. For example for PostsController:

  • post - one Post instance
  • posts - Search Relatation (for lazy load or further scoping)
  • post_list - Array of Post instances

This simplifies the above PostsController example to this:

class PostsController < ApplicationController
  # alternatively: context_exposer :resource
  include ContextExposer::ResourceController

  expose_resources :all

The macro expose_resources optionally takes a list of the types of resource you want to expose. Valid types are :one, :many and :list respectively (for fx: post, posts and post_list).

ContextExposer::ResourceController uses the following internal logic for its default functionality. You can override these methods to customize your behavior as needed.

module ContextExposer::ResourceController
  # ...


  def resource_id

  def find_single_resource
    self.class._the_resource.find resource_id

  def find_all_resources

Tip: You can create reusable module and then include your custom ResourceController.

module NamedResourceController
  extend ActiveSupport::Concern
  include ContextExposer::ResourceController


  def resource_id
class PostsController < ApplicationController
  include NamedResourceController

Tip: If you put your custom module inside the ContextExposer namespace, you can even use the context_exposer macro ;)

Integrations with other exposure gems and patterns

You can use the class macro integrate_with(name) to integrate with either:

  • decent_exposure - integrate_with :decent_exposure
  • decorates_before_rendering - integrate_with :decorates_before
  • instance vars - integrate_with :instance_vars
  • drapper - integrate_with :instance_vars

Note: You can even integrate with multiple strategies

integrate_with :decent_exposure, :instance_vars

You can also specify your integrations directly as part of your context_exposer call (recommended)

context_exposer :base, with: :decent_exposure

In case you use the usual (default) Rails pattern of passing instance variables, you can slowly migrate to exposing via ctx object, by adding a simple macro context_expose :instance_vars to your controller.

For decorated instance variables (see decorates_before_rendering gem), similarly use context_expose :decorated_instance_vars.

All of these context_expose :xxxx methods can optionally take an :except or :only option with a list of keys, similar to a before_filter.

The method context_expose :decorated_instance_vars can additionally take a :for option of either :collection or :non_collection to limit the type of instance vars exposed.

context_expose integration

  • :instance_vars
  • :decorated_instance_vars (decorates_before_rendering)
  • :decently (decent_exposure)
  • :assigned (draper)

Here is a full example demonstrating integration with decent_exposure.

# using gem 'decent_exposure'
# auto-included in ActionController::Base

class PostsController < ApplicationController
  # make context_expose_decently method available
  context_exposer :base, with :decent_exposure

  expose(:posts)  { Post.all.asc(:created_at) }
  expose(:post)   { Post.first}
  expose(:postal) { '1234' }

  # mirror all methods exposed via #expose on #ctx object 
  # except for 'postal' method
  context_expose :decently, except: 'postal'

HAML view example

%h1 Posts
= ctx.posts.each do |post|
  %h2 = post.name


The draper gem adds a decorates_assigned method since version 1.1 (see pull request).

decorates_assigned :article, with: FancyArticleDecorator
decorates_assigned :articles, with: PaginatingCollectionDecorator

Since this functionality is very similar to fx decent_exposure, it can be used with ctx in a similar way. Simply use the context_expose_assigned like the context_expose_decently macro.

context_expose :assigned

`context_expose :assigned, only: %w{post posts}

Decorates before rendering

A patch for the decorates_before_render gem is currently made available.

ContextExposer.patch :decorates_before_rendering

You typically use this in a Rails initializer. This way, decorates_before_rendering should try to decorate all your exposed variables before rendering, whether your view context is exposed as instance vars, methods or on the ctx object of the view ;)

Note: You can also use the macro decorates_before_render to include the DecoratesBeforeRendering module.

Auto-finding a decorator

For the patched version of decorates_before_render to work, your exposed and cached object must either have a model_name method that returns the name of the model name to be used to calculate the decorator name to use, or alternatively (and with higher precedence if present), a decorator method that takes the controller (self) as an argument and returns the full name of the decorator to use ;)


class PostsController < ApplicationController
  context_exposer :base, with :decent_exposure

  expose_cached(:first_post) { Post.first } 


  def admin?
    @admin ||= current_user.admin?

Example: Model with a decorator method to return the class name of the decorator to use.

class Post < ActiveRecord::Base
  def decorator contrl
    contrl.send(:admin?) ? 'Admin::PostDecorator' : model_name      

Error handling for Auto-detection

If the auto-decoration can't find a decorator for an exposed variable (or method), it will either ignore it (not decorate it) or call __handle_decorate_error_(error) which by default will log a Rails warning. Override this error handler as it suits you.

Globalizing the page context

As you have the ctx object encapsulate all the view state in one place, you can simplify your partial calls to render partial: 'my/partial/template', locals: {ctx: ctx}. However, if you use nested partials it quickly gets repetitive and ugly with locals hashes everywhere...

Which is why this pattern is now encapsulated as global view helpers:

  • page_context
  • ctx (data context)
  • page

So you then only have to use the locals hash if you want to pass on variables not part of ctx.

render partial: 'my/partial/template', locals {very_local: 1}

Page object

To further help in making page rendering decissions, a ContextExposer::Page instance is created and populated on each request, which can contain the following data:

:name, :id, :action, :mode, :controller_name, :type, :resource

The Page instance will attempt to calculate the resource name from the normalized_resource_name method of the ContextExposer::BaseController. Override this method to calculate a custom resource name for the controller.

The page name will normally be calculated by concatenating action, resource name and type, so a PostController#show action will have the default name 'show_post_item'. Resource type is either :list or :item and will be attempted calculated using the action name and looking up in the list_actions and item_actions class methods on the controller.

By default these methods will use the base_list_actions (index) and base_item_actions (show, new, edit). You can override/extend these conventions and provide your own list_actions and item_actions class methods for each controller. Macros are provided to generate these methods from a simple list.

class Admin::BirdLocationController < ApplicationController
  # expose, decorate etc left out

  # use macros to configure extra REST-like actions
  list_actions :manage
  item_actions :map

  # custom page object config just before render
  after_filter :set_page_mode

  # manage many birds
  def manage

  # show a single bird location on the map
  def map
    Bird.find params[:id]


  def set_page_mode
    ctx.page.mode = mode

  # custom calculated page name using fx action_name method and params etc
  def page_name

  # map, details or normal mode ?
  def mode

  def self.normalized_resource_name


The tests have been written in rspec 2 and capybara. The test suite consists of:

  • Full app tests
  • Units tests

Dummy app feature tests

A Dummy app has been set up for use with Capybara feature testing. Please see: http://alindeman.github.com/2012/11/11/rspec-rails-and-capybara-2.0-what-you-need-to-know.html

The feature tests can be found in spec/app


$ bundle exec rspec spec/app

  • posts_spec - basic functionality
  • items_spec - cached resource
  • animals_spec - Draper decorator integration


  • Many more app integration tests are needed :P

Unit tests (specs)

The unit tests can be found in spec/context_exposer

$ bundle exec rspec spec/context_exposer


  • Many more unit tests are needed :P


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request