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View components for Rails.


View Components for Rails.


Cells allow you to encapsulate parts of your page into separate MVC components. These components are called view models.

You can render view models anywhere in your code. Mostly, cells are used in views to replace a helper/partial/filter mess, as a mailer renderer substitute or they get hooked to routes to completely bypass ActionController.

As you have already noticed we use cell and view model interchangeably here.

The Book

Cells is part of the Trailblazer project. Please buy my book to support the development and to learn all the cool stuff about Cells. The book discusses the following.

  • Basic view models, replacing helpers, and how to structure your view into cell components (chapter 2 and 4).
  • Advanced Cells API (chapter 4 and 6).
  • Testing Cells (chapter 4 and 6).
  • Cells Pagination with AJAX (chapter 6).
  • View Caching and Expiring (chapter 7).

More chapters are coming.

The book picks up where the README leaves off. Go grab a copy and support us - it talks about object- and view design and covers all aspects of the API.

No ActionView

Starting with Cells 4.0 we no longer use ActionView as a template engine. Removing this jurassic dependency cuts down Cells' rendering code to less than 50 lines and improves rendering speed by 300%!

Note for Cells 3.x: This README only documents Cells 4.0. Please read the old README if you're using Cells 3.x.


Cells run with all Rails >= 3.2. Lower versions of Rails will still run with Cells, but you will get in trouble with the helpers.

gem 'cells', "~> 4.0.0"


Cells comes bundled with ERB support. To render HAML, you have to include the cells-haml gem. The same for cells-slim. Currently, they are only available as github dependencies, they will be released soon (early 2015).

gem "cells-haml", github: 'trailblazer/cells-haml'

The template engine extensions fix severe bugs in combination with Rails helpers and the respective engine. Time will tell if we can convince the template teams to merge these fixes.

File Layout

Cells are placed in app/cells.

├── cells
│   ├── comment_cell.rb
│   ├── comment
│   │   ├── show.haml
│   │   ├── list.haml


Use the bundled generator to set up a cell.

rails generate cell comment
create  app/cells/
create  app/cells/comment
create  app/cells/comment_cell.rb
create  app/cells/comment/show.erb

Rendering View Models

Suppose we are to render a "partial" for Comment model

@comment = Comment.find(1)

Cells brings you one helper method #cell to be used in your controller views or layouts.

= cell(:comment, @comment)

This is the short form of rendering a cell. Simple, isn't it?

Note that a view model always requires a model in the constructor (or a composition). This doesn't have to be an ActiveRecord object but can be any type of Ruby object you want to present.

To understand invoking cells, here's the long form of it.

= cell(:comment, @comment).call(:show)
  1. #cell(..) simply returns the cell instance. You can do whatever you want with it.
  2. .call(:show) will invoke the #show method respecting caching settings.

When rendering cells in views, you can skip the call part as this is implicitely done by the template.

Please refer to the docs for different ways of invoking view models.

View Model Classes

A view model is always implemented as a class. This gives you encapsulation, proper inheritance and namespacing out-of-the-box.

class CommentCell < Cell::ViewModel
  def show

Calling #render will render the cell's show.haml template, located in app/cells/comment. Invoking render is explicit: this means, it really returns the rendered view string, allowing you to modify the HTML afterwards.

def show
  "<div>" + render + "</div>"

Views In Theory

In Cells, we don't distinguish between view or partial. Every view you render is a partial, every partial a view. You can render views inside views, compose complex UI blocks with multiple templates and go crazy. This is what cells views are made for.

Cells supports all template engines that are supported by the excellent tilt gem - namely, this is ERB, HAML, Slim, and many more.

In these examples, we're using HAML.

BTW, Cells doesn't include the format into the view name. 99% of all cells render HTML anyway, so we prefer short names like show.haml.

Views In Practice

Let's check out the show.haml view to see how they work.

-# app/cells/comment/show.haml

%h1 Comment

= model.body
= link_to,

Cells provides you the view model via the #model method. Here, this returns the Comment instance passed into the constructor.

Of course, this view is a mess and needs be get cleaned up!

Logicless Views

This is how a typical view looks in a view model.

-# app/cells/comment/show.haml

%h1 Comment

= body
= author_link

The methods we call in the view now need to be defined in the cell instance.

class CommentCell < Cell::ViewModel
  def show


  def body

  def author_link

See how you can use helpers in a cell instance?

No Helpers

The difference to conventional Rails views is that every method called in a view is directly called on the cell instance. The cell instance is the rendering context. This allows a very object-oriented and clean way to implement views.

Helpers as known from conventional Rails where methods and variables get copied between view and controller no longer exist in Cells.

Note that you can still use helpers like link_to and all the other friends - you have to include them into the cell class, though.

Automatic Properties

Often, as in the #body method, you simply need to delegate properties from the model. This can be done automatically using ::property.

class CommentCell < Cell::ViewModel
  def show

  property :body
  property :author

  def author_link
    link_to, author

Readers are automatically created when defined with ::property.


multiple times allowed :view :format ".html" template_engine view_paths

Invocation styles

The explicit, long form allows you rendering cells in views, in controllers, mailers, etc.

cell(:comment, @comment).call(:show)

As :show is the default action, you don't have to specify it.

cell(:comment, @comment).call

In views, the template engine will automatically call cell.to_s. It does that for every object passed in as a placeholder. ViewModel#to_s exists and is aliased to #call, which allows to omit that part in a view.

= cell(:comment, @comment)

If you want, you can also call public methods directly on your cell. Note that this does not respect caching, though.

= cell(:comment, @comment).avatar

Passing Options

There's several ways to inject additional state into your cell.

Object Style

Cells can receive any set of options you need. Usually, a hash containing additional options is passed as the last argument.

cell(:comment, @comment, layout: :fancy)

The third argument is accessable via #options in the instance.

def show
  render layout: options[:layout]

Functional Style

You can also pass options to the action method itself, making your cell a bit more functional with less state.

cell(:comment, @comment).call(:show, layout: :fancy)

Make sure the method is ready to process those arguments.

def show(layout=:default)
  render layout: layout


You can render a collection of models where each item is rendered using a cell.

= cell(:song, collection: Song.all)

Note that there is no .call needed. This is identical to the following snippet.

- Song.all.each do |song|
  = cell(:song, song).call(:show)

Options are passed to every cell.

= cell(:song, collection: Song.all, genre: "Heavy Metal", user: current_user)

The collection invocation per default calls #show. Use :method if you need another method to be called.

= cell(:song, collection: Song.all, method: :detail)


Often, it is good practice to replace decider code from views or classes into separate sub-cells. Or in case you want to render a polymorphic collection, builders come in handy.

Builders allow instantiating different cell classes for different models and options.

class SongCell < Cell::ViewModel
  builds do |model, options|
    HitCell       if model.is_a?(Hit)
    EverGreenCell if model.is_a?(Evergreen)

  def show
    # ..

The #cell helpers takes care of instantiating the right cell class for you.

cell(:song, Hit.find(1)) #=> creates a HitCell.

This also works with collections.

cell(:song, collection: [@hit, @song]) #=> renders HitCell, then SongCell.

Multiple calls to ::builds will be ORed. If no block returns a class, the original class will be used (SongCell). Builders are inherited.

View Inheritance

TODO: merge stuff below!

File Structure

In Cells 3.10 we introduce a new optional file structure integrating with Trailblazer's "concept-oriented" layout.

This new file layout makes a cell fully self-contained so it can be moved around just by grabbing one single directory.

Activate it with

class Comment::Cell

  # ...

Now, the cell directory ideally looks like the following.

├── cells
│   ├── comment
│   │   ├── cell.rb
│   │   ├── views
│   │   │   ├── show.haml
│   │   │   ├── list.haml

Here, cell class and associated views are in the same self-contained comment directory.

You can use the new views directory along with leaving your cell class at app/cells/comment_cell.rb, if you fancy that.

Asset Pipeline

Cells can also package their own assets like JavaScript, CoffeeScript, Sass and stylesheets. When configured, those files go directly into Rails' asset pipeline. This is a great way to clean up your assets by pushing scripts and styles into the component they belong to. It makes it so much easier to find out which files are actually involved per "widget".

Note: This feature is still experimental and the API (file name conventions, configuration, etc.) might change.

Assets per default sit in the cell's assets/ directory.

├── cells
│   ├── comment
│   │   ├── views
│   │   ├── ..
│   │   ├── assets
│   │   │   ├──
│   │   │   ├── comment.css.sass

Adding the assets files to the asset pipeline currently involves two steps (I know it feels a bit clumsy, but I'm sure we'll find a way to make it better soon).

  1. Tell Rails that this cell provides its own self-contained assets.

    Gemgem::Application.configure do
      # ...
      config.cells.with_assets = %w(comment)

    This will add app/cells/comment/assets/ to the asset pipeline's paths.

  2. Include the assets in application.js and application.css.sass

    In app/assets/application.js, you have to add the cell assets manually.

    //=# require comments

    Same goes into app/assets/application.css.sass.

    @import 'comments';

In future versions, we wanna improve this by automatically including cell assets and avoiding name clashes. If you have ideas, suggestions, I'd love to hear them.

View Inheritance

This is where OOP comes back to your view.

  • Inherit code into your cells by deriving more abstract cells.
  • Inherit views from parent cells.

Sharing Views

Sometimes it is handy to reuse an existing view directory from another cell, to avoid a growing number of directories. You could derive the new cell and thus inherit the view paths.

class Comment::FormCell < CommentCell

This does not only allow view inheritance, but will also inherit all the code from CommentCell. This might not be what you want.

If you're just after inheriting the views, use ::inherit_views.

class Comment::FormCell < Cell::Rails
  inherit_views CommentCell

When rendering views in FormCell, the view directories to look for templates will be inherited.


Cells allow you to cache per state. It's simple: the rendered result of a state method is cached and expired as you configure it.

To cache forever, don't configure anything

class CartCell < Cell::Rails
  cache :show

  def show

This will run #show only once, after that the rendered view comes from the cache.

Cache Options

Note that you can pass arbitrary options through to your cache store. Symbols are evaluated as instance methods, callable objects (e.g. lambdas) are evaluated in the cell instance context allowing you to call instance methods and access instance variables. All arguments passed to your state (e.g. via render_cell) are propagated to the block.

cache :show, :expires_in => 10.minutes

If you need dynamic options evaluated at render-time, use a lambda.

cache :show, :tags => lambda { |*args| tags }

If you don't like blocks, use instance methods instead.

class CartCell < Cell::Rails
  cache :show, :tags => :cache_tags

  def cache_tags(*args)
    # do your magic..

Conditional Caching

The +:if+ option lets you define a condition. If it doesn't return a true value, caching for that state is skipped.

cache :show, :if => lambda { |*| has_changed? }

Cache Keys

You can expand the state's cache key by appending a versioner block to the ::cache call. This way you can expire state caches yourself.

class CartCell < Cell::Rails
  cache :show do |options|

The versioner block is executed in the cell instance context, allowing you to access all stakeholder objects you need to compute a cache key. The return value is appended to the state key: "cells/cart/show/1".

As everywhere in Rails, you can also return an array.

class CartCell < Cell::Rails
  cache :show do |options|
    [id, options[:items].md5]

Resulting in: "cells/cart/show/1/0ecb1360644ce665a4ef".

Debugging Cache

When caching is turned on, you might wanna see notifications. Just like a controller, Cells gives you the following notifications.

  • write_fragment.action_controller for cache miss.
  • read_fragment.action_controller for cache hits.

To activate notifications, include the Notifications module in your cell.

class Comment::Cell < Cell::Rails
  include Cell::Caching::Notifications


Cache configuration is inherited to derived cells.

A Note On Fragment Caching

Fragment caching is not implemented in Cells per design - Cells tries to move caching to the class layer enforcing an object-oriented design rather than cluttering your views with caching blocks.

If you need to cache a part of your view, implement that as another cell state.

Testing Caching

If you want to test it in development, you need to put config.action_controller.perform_caching = true in development.rb to see the effect.


Another big advantage compared to monolithic controller/helper/partial piles is the ability to test your cells isolated.


So what if you wanna test the cart cell? Use the generated test/cells/cart_cell_test.rb test.

class CartCellTest < Cell::TestCase
  test "show" do
    invoke :show, :user => @user_fixture
    assert_select "#cart", "You have 3 items in your shopping cart."

Don't forget to put require 'cell/test_case' in your project's test/test_helper.rb file.

Then, run your tests with

rake test:cells

That's easy, clean and strongly improves your component-driven software quality. How'd you do that with partials?


If you prefer RSpec examples, use the rspec-cells gem for specing.

it "should render the posts count" do
  render_cell(:posts, :count).should have_selector("p", :content => "4 posts!")

To run your specs we got a rake task, too!

rake spec:cells


The #call method also accepts a block and yields self (the cell instance) to it. This is extremely helpful for using content_for outside of the cell.

  = cell(:song, Song.last).call(:show) do |cell|
    content_for :footer, cell.footer

Note how the block is run in the global view's context, allowing you to use global helpers like content_for.

Using Decorators (Twins)

You need to include the disposable gem in order to use this.

gem "disposable"

With Cells 3.12, a new experimental concept enters the stage: Decorators in view models. As the view model should only contain logic related to presentation (which can get quite a bit), decorators - called Twins - can be defined and automatically setup for your model.

Twins are a general concept in Trailblazer and are used everywhere where representers, forms, operations or cells need additional logic that has to be shared between layers. So, this extra step allows re-using your decorator for presentations other than the cell, e.g. in a JSON API, tests, etc.

Also, logic that simply doesn't belong to in a view-related class goes into a twin. That could be code to figure out if a user in logged in.

class SongCell < Cell::ViewModel
  include Properties

  class Twin < Cell::Twin # this is your decorator
    property :title
    property :id
    option :in_stock?

  properties Twin

  def show
    if in_stock?
      "You're lucky #{title} (#{id}) is in stock!"

In this example, we define the twin in the cell itself. That could be done anywhere, as long as you tell the cell where to find the twin (properties Twin).

Creating A Twin Cell

You create your cell as follows.

cell("song", Song.find(1), in_stock?: true)

Internally, a twin is created from the arguments and passed to the view model. The view model cell now only works on the twin, not on the model anymore.

The twin simply acts as a delegator between the cell and the model: attributes defined with property are copied from the model, option values have to be passed explicitely to the constructor. The twin defines an interface for using your cell.

Another awesome thing is that you can now easily test your cell by "mocking" values.

it "renders nicely" do
  cell("song", song, in_stock?: true, title: "Mocked Song Title").must_match ...

The twin will simply use the passed :title and not copy the title from the song model, making it really easy to test edge cases in your view model.

Extending Decorators

A decorator without any logic only gives you a tiny improvement, they become really helpful when including your own decorator logic.

class Twin < Cell::Twin # this is your decorator
  property :title
  property :id
  option :in_stock?

  def title
    super.downcase # super to retrieve the original title from model!

The same logic can now be used in a cell, a JSON or XML API endpoint or in the model layer.

Note: If there's enough interest, this could also be extended to work with draper and other decoration gems.

Nested Rendering

When extracting parts of your view into a partial, as we did for the author section, you're free to render additional views using #render. Again, wrap render calls in instance methods, otherwise you'll end up with too much logic in your view.

class SongCell < Cell::ViewModel
  include TimeagoHelper

  property :title

  # ...

  def author_box
    render :author # same as render view: :author

This will simply render the author.haml template in the same context as the show view, meaning you might use helpers, again.


If in doubt, encapsulate nested parts of your view into a separate cell. You can use the #cell method in your cell to instantiate a nested cell.

Designing view models to create kickass UIs for your domain layer is discussed in 50+ pages in my upcoming book.

Alternative Instantiation

You don't need to pass in a model, it can also be a hash for a composition.

  cell(album, song: song, composer: album.composer)

This will create two readers in the cell for you automatically: #song and #composer.

Note that we are still working on a declarative API for compositions. It will be similar to the one found in Reform, Disposable::Twin and Representable:

  property :title, on: :song
  property :last_name, on: :composer

Mountable Cells

Cells 3.8 got rid of the ActionController dependency. This essentially means you can mount Cells to routes or use them like a Rack middleware. All you need to do is derive from Cell::Base.

class PostCell < Cell::Base

In your routes.rb file, mount the cell like a Rack app.

match "/posts" => proc { |env|
  [ 200, {}, [ Cell::Base.render_cell_for(:post, :show) ]]

Cells in ActionMailer

ActionMailer doesn't have request object, so if you inherit from Cell::Rails you will receive an error. Cell::Base will fix that problem, but you will not be able to use any of routes inside your cells.

You can fix that with actionmailer_with_request which (suprise!) brings request object to the ActionMailer.

Cells is Rails::Engine aware!

Now Rails::Engines can contribute to Cells view paths. By default, any 'app/cells' found inside any Engine is automatically included into Cells view paths. If you need to, you can customize the view paths changing/appending to the 'app/cell_views' path configuration. See the Cell::EngineIntegration for more details.

Generator Options

By default, generated cells inherit from Cell::ViewModel. If you want to change this, specify your new class name in config/application.rb:

Base Class

module MyApp
  class Application < Rails::Application
    config.generators do |g|
      g.base_cell_class "ApplicationCell"

Base Path

You can configure the cells path in case your cells don't reside in app/cells.

config.generators do |g|
  g.base_cell_path "app/widgets"

Capture Support

If you need a global #content_for use the cells-capture gem.

Undocumented Features

(Please don't read this section!)

Rendering Global Partials

Although not recommended, you can also render global partials from a cell. Be warned, though, that they will be rendered using our stack, and you might have to include helpers into your view model.

This works by including Partial and the corresponding :partial option.

class Cell < Cell::ViewModel
  include Partial

  def show
    render partial: "../views/shared/map.html" # app/views/shared/map.html.haml

The provided path is relative to your cell's ::view_paths directory. The format has to be added to the file name, the template engine suffix will be used from the cell.

You can provide the format in the render call, too.

render partial: "../views/shared/map", formats: [:html]

This was mainly added to provide compatibility with 3rd-party gems like Kaminari and Cells that rely on rendering partials within a cell.


Copyright (c) 2007-2015, Nick Sutterer

Copyright (c) 2007-2008, Solide ICT by Peter Bex and Bob Leers

Released under the MIT License.

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