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View Components for Rails.


Say you're writing a Rails online shop - the shopping cart is reappearing again and again in every view. You're thinking about a clean solution for that part. A mixture of controller code, before-filters, partials and helpers?

No. That sucks. Take Cells.

Cells are View Components for Rails. They look and feel like controllers. They don't have no DoubleRenderError. They can be rendered everywhere in your controllers or views. They are cacheable, testable, fast and wonderful. They bring back OOP to your view and improve your software design.

And the best: You can have as many cells in your page as you need!


It's a gem!

Rails 3.x:

gem install cells

Rails 2.3:

gem install cells -v 3.3.9


Creating a cell is nothing more than

$ rails generate cell cart display -e haml
  create  app/cells/
  create  app/cells/cart
  create  app/cells/cart_cell.rb
  create  app/cells/cart/display.html.haml
  create  test/cells/cart_test.rb

That looks very familiar.

Render the cell

Now, render your cart. Why not put it in layouts/application.html.erb for now?

<div id="header">
  <%= render_cell :cart, :display, :user => @current_user %>

Feels like rendering a controller action. For good encapsulation we pass the current user from outside into the cell - a dependency injection.


Time to improve our cell code. Let's start with app/cells/cart_cell.rb:

class CartCell < Cell::Rails
  def display(args)
    user    = args[:user]
    @items  = user.items_in_cart

    render  # renders display.html.haml

Is that a controller? Hell, yeah. We even got a #render method as we know it from the good ol' ActionController.


Since a plain call to #render will start rendering app/cells/cart/display.html.haml we should put some meaningful markup there.

  You have #{@items.size} items in your shopping cart.

ERB? Haml? Builder?

Yes, Cells support all template types that are supported by Rails itself. Remember- it's a controller!


Yes, Cells have helpers just like controllers. If you need some specific helper, do

class CartCell < Cell::Rails
  helper MyExtraHelper

and it will be around in your cart views.


Yeah, we do support rendering partials in views. Nevertheless, we discourage partials at all.

The distinction between partials and views is making things more complex, so why should we have two kinds of view types? Use ordinary views instead, they're fine.

  = render :view => 'items'

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.


Let render_cell take care of creating the right cell. Just configure your super-cell properly.

class LoginCell < Cell::Rails
  build do
    UnauthorizedUserCell unless logged_in?

A call to

render_cell(:login, :box)

will render the configured UnauthorizedUserCell instead of the original LoginCell if the login test fails.


Cells do strict view caching. No cluttered fragment caching. Add

class CartCell < Cell::Rails
  cache :display, :expires_in => 10.minutes

and your cart will be re-rendered after 10 minutes.

You can expand the state's cache key - why not use a versioner block to do just this?

class CartCell < Cell::Rails
  cache :display do |cell, options|

The block's return value is appended to the state key: "cells/cart/display/0ecb1360644ce665a4ef".

Check the API to learn more.


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 "display" do
    invoke :display, :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

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 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.

Rails 2.3 note

In order to copy the cells rake tasks to your app, run

$ script/generate cells_install

More features

Cells can do more.

No Limits

Have as many cells in your page as you need - no limitation to your render_cell calls.

Cell Nesting

Have complex cell hierarchies as you can call render_cell within cells, too.

Go for it, you'll love it!


Copyright © 2007-2011, Nick Sutterer

Copyright © 2007-2008, Solide ICT by Peter Bex and Bob Leers

Released under the MIT License.