Guides.rubyonrails.org

Chapters

1. Overview: How the Pieces Fit Together
2. Creating Responses
   • Rendering by Default: Convention Over Configuration in Action
   • Using render
   • Using redirect_to
   • Using head To Build Header-Only Responses
3. Structuring Layouts
   • Asset Tags
   • Understanding yield
   • Using content_for
   • Using Partials
   • Using Nested Layouts
4. Changelog

1 Overview: How the Pieces Fit Together

This guide focuses on the interaction between Controller and View in the Model-View-Controller triangle. As you know, the Controller is responsible for orchestrating the whole process of handling a request in Rails, though it normally hands off any heavy code to the Model. But then, when it’s time to send a response back to the user, the Controller hands things off to the View. It’s that handoff that is the subject of this guide.

In broad strokes, this involves deciding what should be sent as the response and calling an appropriate method to create that response. If the response is a full-blown view, Rails also does some extra work to wrap the view in a layout and possibly to pull in partial views. You’ll see all of those paths later in this guide.

2 Creating Responses

From the controller’s point of view, there are three ways to create an HTTP response:

• Call render to create a full response to send back to the browser
• Call redirect_to to send an HTTP redirect status code to the browser
• Call head to create a response consisting solely of HTTP headers to send back to the browser

I’ll cover each of these methods in turn. But first, a few words about the very easiest thing that the controller can do to create a response: nothing at all.

2.1 Rendering by Default: Convention Over Configuration in Action

You’ve heard that Rails promotes “convention over configuration”. Default rendering is an excellent example of this. By default, controllers in Rails automatically render views with names that correspond to valid routes. For example, if you have this code in your BooksController class:

class BooksController < ApplicationController
end

And the following in your routes file:

resources :books

And you have a view file app/views/books/index.html.erb:

<h1>Books are coming soon!</h1>

Rails will automatically render app/views/books/index.html.erb when you navigate to /books and you will see “Books are coming soon!” on your screen.

However a coming soon screen is only minimally useful, so you will soon create your Book model and add the index action to BooksController:

class BooksController < ApplicationController
  def index
    @books = Book.all
  end
end

Note that we don’t have explicit render at the end of the index action in accordance with “convention over configuration” principle. The rule is that if you do not explicitly render something at the end of a controller action, Rails will automatically look for the action_name.html.erb template in the controller’s view path and render it. So in this case, Rails will render the app/views/books/index.html.erb file.

If we want to display the properties of all the books in our view, we can do so with an ERB template like this:

<h1>Listing Books</h1>

<table>
  <tr>
    <th>Title</th>
    <th>Summary</th>
    <th></th>
    <th></th>
    <th></th>
  </tr>

<% @books.each do |book| %>
  <tr>
    <td><%= book.title %></td>
    <td><%= book.content %></td>
    <td><%= link_to 'Show', book %></td>
    <td><%= link_to 'Edit', edit_book_path(book) %></td>
    <td><%= link_to 'Remove', book, :confirm => 'Are you sure?', :method => :delete %></td>
  </tr>
<% end %>
</table>

<br />

<%= link_to 'New book', new_book_path %>

2.2 Using render

In most cases, the ActionController::Base#render method does the heavy lifting of rendering your application’s content for use by a browser. There are a variety of ways to customize the behaviour of render. You can render the default view for a Rails template, or a specific template, or a file, or inline code, or nothing at all. You can render text, JSON, or XML. You can specify the content type or HTTP status of the rendered response as well.

2.2.1 Rendering Nothing

Perhaps the simplest thing you can do with render is to render nothing at all:

render :nothing => true

If you look at the response for this using cURL, you will see the following:

$ curl -i 127.0.0.1:3000/books
HTTP/1.1 200 OK
Connection: close
Date: Sun, 24 Jan 2010 09:25:18 GMT
Transfer-Encoding: chunked
Content-Type: */*; charset=utf-8
X-Runtime: 0.014297
Set-Cookie: _blog_session=...snip...; path=/; HttpOnly
Cache-Control: no-cache


 $

We see there is an empty response (no data after the Cache-Control line), but the request was successful because Rails has set the response to 200 OK. You can set the :status option on render to change this response. Rendering nothing can be useful for AJAX requests where all you want to send back to the browser is an acknowledgment that the request was completed.

2.2.2 Rendering an Action’s View

If you want to render the view that corresponds to a different action within the same template, you can use render with the name of the view:

def update
  @book = Book.find(params[:id])
  if @book.update_attributes(params[:book])
    redirect_to(@book)
  else
    render "edit"
  end
end

If the call to update_attributes fails, calling the update action in this controller will render the edit.html.erb template belonging to the same controller.

If you prefer, you can use a symbol instead of a string to specify the action to render:

def update
  @book = Book.find(params[:id])
  if @book.update_attributes(params[:book])
    redirect_to(@book)
  else
    render :edit
  end
end

To be explicit, you can use render with the :action option (though this is no longer necessary in Rails 3.0):

def update
  @book = Book.find(params[:id])
  if @book.update_attributes(params[:book])
    redirect_to(@book)
  else
    render :action => "edit"
  end
end

2.2.3 Rendering an Action’s Template from Another Controller

What if you want to render a template from an entirely different controller from the one that contains the action code? You can also do that with render, which accepts the full path (relative to app/views) of the template to render. For example, if you’re running code in an AdminProductsController that lives in app/controllers/admin, you can render the results of an action to a template in app/views/products this way:

render 'products/show'

Rails knows that this view belongs to a different controller because of the embedded slash character in the string. If you want to be explicit, you can use the :template option (which was required on Rails 2.2 and earlier):

render :template => 'products/show'

2.2.4 Rendering an Arbitrary File

The render method can also use a view that’s entirely outside of your application (perhaps you’re sharing views between two Rails applications):

render "/u/apps/warehouse_app/current/app/views/products/show"

Rails determines that this is a file render because of the leading slash character. To be explicit, you can use the :file option (which was required on Rails 2.2 and earlier):

render :file =>
  "/u/apps/warehouse_app/current/app/views/products/show"

The :file option takes an absolute file-system path. Of course, you need to have rights to the view that you’re using to render the content.

2.2.5 Wrapping it up

The above three ways of rendering (rendering another template within the controller, rendering a template within another controller and rendering an arbitrary file on the file system) are actually variants of the same action.

In fact, in the BooksController class, inside of the update action where we want to render the edit template if the book does not update successfully, all of the following render calls would all render the edit.html.erb template in the views/books directory:

render :edit
render :action => :edit
render 'edit'
render 'edit.html.erb'
render :action => 'edit'
render :action => 'edit.html.erb'
render 'books/edit'
render 'books/edit.html.erb'
render :template => 'books/edit'
render :template => 'books/edit.html.erb'
render '/path/to/rails/app/views/books/edit'
render '/path/to/rails/app/views/books/edit.html.erb'
render :file => '/path/to/rails/app/views/books/edit'
render :file => '/path/to/rails/app/views/books/edit.html.erb'

Which one you use is really a matter of style and convention, but the rule of thumb is to use the simplest one that makes sense for the code you are writing.

2.2.6 Using render with :inline

The render method can do without a view completely, if you’re willing to use the :inline option to supply ERB as part of the method call. This is perfectly valid:

render :inline =>
  "<% products.each do |p| %><p><%= p.name %></p><% end %>"

By default, inline rendering uses ERB. You can force it to use Builder instead with the :type option:

render :inline =>
  "xml.p {'Horrid coding practice!'}", :type => :builder

2.2.7 Rendering Text

You can send plain text – with no markup at all – back to the browser by using the :text option to render:

render :text => "OK"

2.2.8 Rendering JSON

JSON is a JavaScript data format used by many AJAX libraries. Rails has built-in support for converting objects to JSON and rendering that JSON back to the browser:

render :json => @product

2.2.9 Rendering XML

Rails also has built-in support for converting objects to XML and rendering that XML back to the caller:

render :xml => @product

2.2.10 Rendering Vanilla JavaScript

Rails can render vanilla JavaScript:

render :js => "alert('Hello Rails');"

This will send the supplied string to the browser with a MIME type of text/javascript.

2.2.11 Options for render

Calls to the render method generally accept four options:

• :content_type
• :layout
• :status
• :location

2.2.11.1 The :content_type Option

By default, Rails will serve the results of a rendering operation with the MIME content-type of text/html (or application/json if you use the :json option, or application/xml for the :xml option.). There are times when you might like to change this, and you can do so by setting the :content_type option:

render :file => filename, :content_type => 'application/rss'

2.2.11.2 The :layout Option

With most of the options to render, the rendered content is displayed as part of the current layout. You’ll learn more about layouts and how to use them later in this guide.

You can use the :layout option to tell Rails to use a specific file as the layout for the current action:

render :layout => 'special_layout'

You can also tell Rails to render with no layout at all:

render :layout => false

2.2.11.3 The :status Option

Rails will automatically generate a response with the correct HTTP status code (in most cases, this is 200 OK). You can use the :status option to change this:

render :status => 500
render :status => :forbidden

Rails understands both numeric status codes and symbols for status codes.

2.2.11.4 The :location Option

You can use the :location option to set the HTTP Location header:

render :xml => photo, :location => photo_url(photo)

2.2.12 Finding Layouts

To find the current layout, Rails first looks for a file in app/views/layouts with the same base name as the controller. For example, rendering actions from the PhotosController class will use app/views/layouts/photos.html.erb (or app/views/layouts/photos.builder). If there is no such controller-specific layout, Rails will use app/views/layouts/application.html.erb or app/views/layouts/application.builder. If there is no .erb layout, Rails will use a .builder layout if one exists. Rails also provides several ways to more precisely assign specific layouts to individual controllers and actions.

2.2.12.1 Specifying Layouts on a per-Controller Basis

You can override the automatic layout conventions in your controllers by using the layout declaration in the controller. For example:

class ProductsController < ApplicationController
  layout "inventory"
  #...
end

With this declaration, all methods within ProductsController will use app/views/layouts/inventory.html.erb for their layout.

To assign a specific layout for the entire application, use a declaration in your ApplicationController class:

class ApplicationController < ActionController::Base
  layout "main"
  #...
end

With this declaration, all views in the entire application will use app/views/layouts/main.html.erb for their layout.

2.2.12.2 Choosing Layouts at Runtime

You can use a symbol to defer the choice of layout until a request is processed:

class ProductsController < ApplicationController
  layout :products_layout

  def show
    @product = Product.find(params[:id])
  end

  private
    def products_layout
      @current_user.special? ? "special" : "products"
    end

end

Now, if the current user is a special user, they’ll get a special layout when viewing a product. You can even use an inline method to determine the layout:

You can also decide the layout by passing a Proc object, the block you give the Proc will be given the controller instance, so you can make decisions based on the current request. For example:

class ProductsController < ApplicationController
  layout Proc.new { |controller| controller.request.xhr? ? 'popup' : 'application' }
end

2.2.12.3 Conditional Layouts

Layouts specified at the controller level support :only and :except options that take either a method name or an array of method names which correspond to method names within the controller:

class ProductsController < ApplicationController
  layout "product", :except => [:index, :rss]
end

With this declaration, the product layout would be used for everything but the rss and index methods.

2.2.12.4 Layout Inheritance

Layouts are shared downwards in the hierarchy, and more specific layouts always override more general ones. For example:

• application_controller.rb

class ApplicationController < ActionController::Base
  layout "main"
end

• posts_controller.rb

class PostsController < ApplicationController
end

• special_posts_controller.rb

class SpecialPostsController < PostsController
  layout "special"
end

• old_posts_controller.rb

class OldPostsController < SpecialPostsController
  layout nil

  def show
    @post = Post.find(params[:id])
  end

  def index
    @old_posts = Post.older
    render :layout => "old"
  end
  # ...
end

In this application:

• In general, views will be rendered in the main layout
• PostsController#index will use the main layout
• SpecialPostsController#index will use the special layout
• OldPostsController#show will use no layout at all
• OldPostsController#index will use the old layout

2.2.13 Avoiding Double Render Errors

Sooner or later, most Rails developers will see the error message “Can only render or redirect once per action”. While this is annoying, it’s relatively easy to fix. Usually it happens because of a fundamental misunderstanding of the way that render works.

For example, here’s some code that will trigger this error:

def show
  @book = Book.find(params[:id])
  if @book.special?
    render :action => "special_show"
  end
  render :action => "regular_show"
end

If @book.special? evaluates to true, Rails will start the rendering process to dump the @book variable into the special_show view. But this will not stop the rest of the code in the show action from running, and when Rails hits the end of the action, it will start to render the regular_show view – and throw an error. The solution is simple: make sure that you have only one call to render or redirect in a single code path. One thing that can help is and return. Here’s a patched version of the method:

def show
  @book = Book.find(params[:id])
  if @book.special?
    render :action => "special_show" and return
  end
  render :action => "regular_show"
end

Make sure you use and return and not && return because while the former will work, the latter will not due to operator precedence in the Ruby Language.

Note that the implicit render done by ActionController detects if render has been called, and thus avoids this error. Therefore, the following will work without errors:

def show
  @book = Book.find(params[:id])
  if @book.special?
    render :action => "special_show"
  end
end

This will render a book with special? set with the special_show template, while other books will render with the default show template.

2.3 Using redirect_to

Another way to handle returning responses to an HTTP request is with redirect_to. As you’ve seen, render tells Rails which view (or other asset) to use in constructing a response. The redirect_to method does something completely different: it tells the browser to send a new request for a different URL. For example, you could redirect from wherever you are in your code to the index of photos in your application with this call:

redirect_to photos_path

You can use redirect_to with any arguments that you could use with link_to or url_for. In addition, there’s a special redirect that sends the user back to the page they just came from:

redirect_to :back

2.3.1 Getting a Different Redirect Status Code

Rails uses HTTP status code 302 (temporary redirect) when you call redirect_to. If you’d like to use a different status code (perhaps 301, permanent redirect), you can do so by using the :status option:

redirect_to photos_path, :status => 301

Just like the :status option for render, :status for redirect_to accepts both numeric and symbolic header designations.

2.3.2 The Difference Between render and redirect_to

Sometimes inexperienced developers conceive of redirect_to as a sort of goto command, moving execution from one place to another in your Rails code. This is not correct. Your code stops running and waits for a new request for the browser. It just happens that you’ve told the browser what request it should make next, by sending back an HTTP 302 status code.

Consider these actions to see the difference:

def index
  @books = Book.all
end

def show
  @book = Book.find_by_id(params[:id])
  if @book.nil?
    render :action => "index"
  end
end

With the code in this form, there will likely be a problem if the @book variable is nil. Remember, a render :action doesn’t run any code in the target action, so nothing will set up the @books variable that the index view is presumably depending on. One way to fix this is to redirect instead of rendering:

def index
  @books = Book.all
end

def show
  @book = Book.find_by_id(params[:id])
  if @book.nil?
    redirect_to :action => :index
  end
end

With this code, the browser will make a fresh request for the index page, the code in the index method will run, and all will be well.

The only downside to this code, is that it requires a round trip to the browser, the browser requested the show action with /books/1 and the controller finds that there are no books, so the controller sends out a 302 redirect response to the browser telling it to go to /books/, the browser complies and sends a new request back to the controller asking now for the index action, the controller then gets all the books in the database and renders the index template, sending it back down to the browser which then shows it on your screen.

While in a small app, this added latency might not be a problem, it is something to think about when speed of response is of the essence. One way to handle this double request (though a contrived example) could be:

def index
  @books = Book.all
end

def show
  @book = Book.find_by_id(params[:id])
  if @book.nil?
    @books = Book.all
    render "index", :alert => 'Your book was not found!'
  end
end

Which would detect that there are no books, populate the @books instance variable with all the books in the database and then directly render the index.html.erb template returning it to the browser with a flash alert message telling the user what happened.

2.4 Using head To Build Header-Only Responses

The head method can be used to send responses with only headers to the browser. It provides a more obvious alternative to calling render :nothing. The head method takes one parameter, which is interpreted as a hash of header names and values. For example, you can return only an error header:

head :bad_request

Which would produce the following header:

HTTP/1.1 400 Bad Request
Connection: close
Date: Sun, 24 Jan 2010 12:15:53 GMT
Transfer-Encoding: chunked
Content-Type: text/html; charset=utf-8
X-Runtime: 0.013483
Set-Cookie: _blog_session=...snip...; path=/; HttpOnly
Cache-Control: no-cache

Or you can use other HTTP headers to convey additional information:

head :created, :location => photo_path(@photo)

Which would produce:

HTTP/1.1 201 Created
Connection: close
Date: Sun, 24 Jan 2010 12:16:44 GMT
Transfer-Encoding: chunked
Location: /photos/1
Content-Type: text/html; charset=utf-8
X-Runtime: 0.083496
Set-Cookie: _blog_session=...snip...; path=/; HttpOnly
Cache-Control: no-cache

3 Structuring Layouts

When Rails renders a view as a response, it does so by combining the view with the current layout (using the rules for finding the current layout that were covered earlier in this guide). Within a layout, you have access to three tools for combining different bits of output to form the overall response:

• Asset tags
• yield and content_for
• Partials

3.1 Asset Tags

Asset tags provide methods for generating HTML that links views to feeds, JavaScript, stylesheets, images, videos and audios. These are the six asset tags available in Rails:

• auto_discovery_link_tag
• javascript_include_tag
• stylesheet_link_tag
• image_tag
• video_tag
• audio_tag

You can use these tags in layouts or other views, although the tags other than image_tag are most commonly used in the <head> section of a layout.

3.1.1 Linking to Feeds with auto_discovery_link_tag

The auto_discovery_link_tag helper builds HTML that most browsers and newsreaders can use to detect the presences of RSS or ATOM feeds. It takes the type of the link (:rss or :atom), a hash of options that are passed through to url_for, and a hash of options for the tag:

<%= auto_discovery_link_tag(:rss, {:action => "feed"},
  {:title => "RSS Feed"}) %>

There are three tag options available for auto_discovery_link_tag:

• :rel specifies the rel value in the link (defaults to “alternate”)
• :type specifies an explicit MIME type. Rails will generate an appropriate MIME type automatically.
• :title specifies the title of the link

3.1.2 Linking to JavaScript Files with javascript_include_tag

The javascript_include_tag helper returns an HTML script tag for each source provided. Rails looks in public/javascripts for these files by default, but you can specify a full path relative to the document root, or a URL, if you prefer. For example, to include public/javascripts/main.js:

<%= javascript_include_tag "main" %>

To include public/javascripts/main.js and public/javascripts/columns.js:

<%= javascript_include_tag "main", "columns" %>

To include public/javascripts/main.js and public/photos/columns.js:

<%= javascript_include_tag "main", "/photos/columns" %>

To include http://example.com/main.js:

<%= javascript_include_tag "http://example.com/main.js" %>

If the application does not use the asset pipeline, the :defaults option loads jQuery by default:

<%= javascript_include_tag :defaults %>

And you can in any case override the expansion in config/application.rb:

config.action_view.javascript_expansions[:defaults] = %w(foo.js bar.js)

When using :defaults, if an application.js file exists in public/javascripts it will be included as well at then end.

Also, the :all option loads every JavaScript file in public/javascripts:

<%= javascript_include_tag :all %>

Note that your defaults of choice will be included first, so they will be available to all subsequently included files.

You can supply the :recursive option to load files in subfolders of public/javascripts as well:

<%= javascript_include_tag :all, :recursive => true %>

If you’re loading multiple JavaScript files, you can create a better user experience by combining multiple files into a single download. To make this happen in production, specify :cache => true in your javascript_include_tag:

<%= javascript_include_tag "main", "columns", :cache => true %>

By default, the combined file will be delivered as javascripts/all.js. You can specify a location for the cached asset file instead:

<%= javascript_include_tag "main", "columns",
  :cache => 'cache/main/display' %>

You can even use dynamic paths such as cache/#{current_site}/main/display.

3.1.3 Linking to CSS Files with stylesheet_link_tag

The stylesheet_link_tag helper returns an HTML <link> tag for each source provided. Rails looks in public/stylesheets for these files by default, but you can specify a full path relative to the document root, or a URL, if you prefer. For example, to include public/stylesheets/main.css:

<%= stylesheet_link_tag "main" %>

To include public/stylesheets/main.css and public/stylesheets/columns.css:

<%= stylesheet_link_tag "main", "columns" %>

To include public/stylesheets/main.css and public/photos/columns.css:

<%= stylesheet_link_tag "main", "/photos/columns" %>

To include http://example.com/main.css:

<%= stylesheet_link_tag "http://example.com/main.css" %>

By default, stylesheet_link_tag creates links with media="screen" rel="stylesheet" type="text/css". You can override any of these defaults by specifying an appropriate option (:media, :rel, or :type):

<%= stylesheet_link_tag "main_print", :media => "print" %>

The all option links every CSS file in public/stylesheets:

<%= stylesheet_link_tag :all %>

You can supply the :recursive option to link files in subfolders of public/stylesheets as well:

<%= stylesheet_link_tag :all, :recursive => true %>

If you’re loading multiple CSS files, you can create a better user experience by combining multiple files into a single download. To make this happen in production, specify :cache => true in your stylesheet_link_tag:

<%= stylesheet_link_tag "main", "columns", :cache => true %>

By default, the combined file will be delivered as stylesheets/all.css. You can specify a location for the cached asset file instead:

<%= stylesheet_link_tag "main", "columns",
  :cache => 'cache/main/display' %>

You can even use dynamic paths such as cache/#{current_site}/main/display.

3.1.4 Linking to Images with image_tag

The image_tag helper builds an HTML <img /> tag to the specified file. By default, files are loaded from public/images.

<%= image_tag "header.png" %>

You can supply a path to the image if you like:

<%= image_tag "icons/delete.gif" %>

You can supply a hash of additional HTML options:

<%= image_tag "icons/delete.gif", {:height => 45} %>

You can also supply an alternate image to show on mouseover:

<%= image_tag "home.gif", :onmouseover => "menu/home_highlight.gif" %>

You can supply alternate text for the image which will be used if the user has images turned off in their browser. If you do not specify an alt text explicitly, it defaults to the file name of the file, capitalized and with no extension. For example, these two image tags would return the same code:

<%= image_tag "home.gif" %>
<%= image_tag "home.gif", :alt => "Home" %>

You can also specify a special size tag, in the format “{width}x{height}”:

<%= image_tag "home.gif", :size => "50x20" %>

In addition to the above special tags, you can supply a final hash of standard HTML options, such as :class, :id or :name:

<%= image_tag "home.gif", :alt => "Go Home",
                          :id => "HomeImage",
                          :class => 'nav_bar' %>

3.1.5 Linking to Videos with video_tag

The video_tag helper builds an HTML 5 <video> tag to the specified file. By default, files are loaded from public/videos.

<%= video_tag "movie.ogg" %>

Produces

<video src="/videos/movie.ogg" />

Like an image_tag you can supply a path, either absolute, or relative to the public/videos directory. Additionally you can specify the :size => "#{width}x#{height}" option just like an image_tag. Video tags can also have any of the HTML options specified at the end (id, class et al).

The video tag also supports all of the <video> HTML options through the HTML options hash, including:

• :poster => 'image_name.png', provides an image to put in place of the video before it starts playing.
• :autoplay => true, starts playing the video on page load.
• :loop => true, loops the video once it gets to the end.
• :controls => true, provides browser supplied controls for the user to interact with the video.
• :autobuffer => true, the video will pre load the file for the user on page load.

You can also specify multiple videos to play by passing an array of videos to the video_tag:

<%= video_tag ["trailer.ogg", "movie.ogg"] %>

This will produce:

<video><source src="trailer.ogg" /><source src="movie.ogg" /></video>

3.1.6 Linking to Audio files with audio_tag

The audio_tag helper builds an HTML 5 <audio> tag to the specified file. By default, files are loaded from public/audios.

<%= audio_tag "music.mp3" %>

You can supply a path to the audio file if you like:

<%= audio_tag "music/first_song.mp3" %>

You can also supply a hash of additional options, such as :id, :class etc.

Like the video_tag, the audio_tag has special options:

• :autoplay => true, starts playing the audio on page load
• :controls => true, provides browser supplied controls for the user to interact with the audio.
• :autobuffer => true, the audio will pre load the file for the user on page load.

3.2 Understanding yield

Within the context of a layout, yield identifies a section where content from the view should be inserted. The simplest way to use this is to have a single yield, into which the entire contents of the view currently being rendered is inserted:

<html>
  <head>
  </head>
  <body>
  <%= yield %>
  </body>
</html>

You can also create a layout with multiple yielding regions:

<html>
  <head>
  <%= yield :head %>
  </head>
  <body>
  <%= yield %>
  </body>
</html>

The main body of the view will always render into the unnamed yield. To render content into a named yield, you use the content_for method.

3.3 Using content_for

The content_for method allows you to insert content into a named yield block in your layout. For example, this view would work with the layout that you just saw:

<% content_for :head do %>
  <title>A simple page</title>
<% end %>

<p>Hello, Rails!</p>

The result of rendering this page into the supplied layout would be this HTML:

<html>
  <head>
  <title>A simple page</title>
  </head>
  <body>
  <p>Hello, Rails!</p>
  </body>
</html>

The content_for method is very helpful when your layout contains distinct regions such as sidebars and footers that should get their own blocks of content inserted. It’s also useful for inserting tags that load page-specific JavaScript or css files into the header of an otherwise generic layout.

3.4 Using Partials

Partial templates – usually just called “partials” – are another device for breaking the rendering process into more manageable chunks. With a partial, you can move the code for rendering a particular piece of a response to its own file.

3.4.1 Naming Partials

To render a partial as part of a view, you use the render method within the view:

<%= render "menu" %>

This will render a file named _menu.html.erb at that point within the view being rendered. Note the leading underscore character: partials are named with a leading underscore to distinguish them from regular views, even though they are referred to without the underscore. This holds true even when you’re pulling in a partial from another folder:

<%= render "shared/menu" %>

That code will pull in the partial from app/views/shared/_menu.html.erb.

3.4.2 Using Partials to Simplify Views

One way to use partials is to treat them as the equivalent of subroutines: as a way to move details out of a view so that you can grasp what’s going on more easily. For example, you might have a view that looked like this:

<%= render "shared/ad_banner" %>

<h1>Products</h1>

<p>Here are a few of our fine products:</p>
...

<%= render "shared/footer" %>

Here, the _ad_banner.html.erb and _footer.html.erb partials could contain content that is shared among many pages in your application. You don’t need to see the details of these sections when you’re concentrating on a particular page.

3.4.3 Partial Layouts

A partial can use its own layout file, just as a view can use a layout. For example, you might call a partial like this:

<%= render :partial => "link_area", :layout => "graybar" %>

This would look for a partial named _link_area.html.erb and render it using the layout _graybar.html.erb. Note that layouts for partials follow the same leading-underscore naming as regular partials, and are placed in the same folder with the partial that they belong to (not in the master layouts folder).

Also note that explicitly specifying :partial is required when passing additional options such as :layout.

3.4.4 Passing Local Variables

You can also pass local variables into partials, making them even more powerful and flexible. For example, you can use this technique to reduce duplication between new and edit pages, while still keeping a bit of distinct content:

• new.html.erb

<h1>New zone</h1>
<%= error_messages_for :zone %>
<%= render :partial => "form", :locals => { :zone => @zone } %>

• edit.html.erb

<h1>Editing zone</h1>
<%= error_messages_for :zone %>
<%= render :partial => "form", :locals => { :zone => @zone } %>

• _form.html.erb

<%= form_for(zone) do |f| %>
  <p>
    <b>Zone name</b><br />
    <%= f.text_field :name %>
  </p>
  <p>
    <%= f.submit %>
  </p>
<% end %>

Although the same partial will be rendered into both views, Action View’s submit helper will return “Create Zone” for the new action and “Update Zone” for the edit action.

Every partial also has a local variable with the same name as the partial (minus the underscore). You can pass an object in to this local variable via the :object option:

<%= render :partial => "customer", :object => @new_customer %>

Within the customer partial, the customer variable will refer to @new_customer from the parent view.

If you have an instance of a model to render into a partial, you can use a shorthand syntax:

<%= render @customer %>

Assuming that the @customer instance variable contains an instance of the Customer model, this will use _customer.html.erb to render it and will pass the local variable customer into the partial which will refer to the @customer instance variable in the parent view.

3.4.5 Rendering Collections

Partials are very useful in rendering collections. When you pass a collection to a partial via the :collection option, the partial will be inserted once for each member in the collection:

• index.html.erb

<h1>Products</h1>
<%= render :partial => "product", :collection => @products %>

• _product.html.erb

<p>Product Name: <%= product.name %></p>

When a partial is called with a pluralized collection, then the individual instances of the partial have access to the member of the collection being rendered via a variable named after the partial. In this case, the partial is _product, and within the _product partial, you can refer to product to get the instance that is being rendered.

In Rails 3.0, there is also a shorthand for this. Assuming @products is a collection of product instances, you can simply write this in the index.html.erb to produce the same result:

<h1>Products</h1>
<%= render @products %>

Rails determines the name of the partial to use by looking at the model name in the collection. In fact, you can even create a heterogeneous collection and render it this way, and Rails will choose the proper partial for each member of the collection:

• index.html.erb

<h1>Contacts</h1>
<%= render [customer1, employee1, customer2, employee2] %>

• customers/_customer.html.erb

<p>Customer: <%= customer.name %></p>

• employees/_employee.html.erb

<p>Employee: <%= employee.name %></p>

In this case, Rails will use the customer or employee partials as appropriate for each member of the collection.

3.4.6 Local Variables

To use a custom local variable name within the partial, specify the :as option in the call to the partial:

<%= render :partial => "product", :collection => @products, :as => :item %>

With this change, you can access an instance of the @products collection as the item local variable within the partial.

You can also pass in arbitrary local variables to any partial you are rendering with the :locals => {} option:

<%= render :partial => 'products', :collection => @products,
           :as => :item, :locals => {:title => "Products Page"} %>

Would render a partial _products.html.erb once for each instance of product in the @products instance variable passing the instance to the partial as a local variable called item and to each partial, make the local variable title available with the value Products Page.

You can also specify a second partial to be rendered between instances of the main partial by using the :spacer_template option:

3.4.7 Spacer Templates

<%= render @products, :spacer_template => "product_ruler" %>

Rails will render the _product_ruler partial (with no data passed in to it) between each pair of _product partials.

3.5 Using Nested Layouts

You may find that your application requires a layout that differs slightly from your regular application layout to support one particular controller. Rather than repeating the main layout and editing it, you can accomplish this by using nested layouts (sometimes called sub-templates). Here’s an example:

Suppose you have the following ApplicationController layout:

• app/views/layouts/application.html.erb

<html>
<head>
  <title><%= @page_title or 'Page Title' %></title>
  <%= stylesheet_link_tag 'layout' %>
  <style type="text/css"><%= yield :stylesheets %></style>
</head>
<body>
  <div id="top_menu">Top menu items here</div>
  <div id="menu">Menu items here</div>
  <div id="content"><%= content_for?(:content) ? yield(:content) : yield %></div>
</body>
</html>

On pages generated by NewsController, you want to hide the top menu and add a right menu:

• app/views/layouts/news.html.erb

<% content_for :stylesheets do %>
  #top_menu {display: none}
  #right_menu {float: right; background-color: yellow; color: black}
<% end %>
<% content_for :content do %>
  <div id="right_menu">Right menu items here</div>
  <%= content_for?(:news_content) ? yield(:news_content) : yield %>
<% end %>
<%= render :template => 'layouts/application' %>

That’s it. The News views will use the new layout, hiding the top menu and adding a new right menu inside the “content” div.

There are several ways of getting similar results with different sub-templating schemes using this technique. Note that there is no limit in nesting levels. One can use the ActionView::render method via render :template => 'layouts/news' to base a new layout on the News layout. If you are sure you will not subtemplate the News layout, you can replace the content_for?(:news_content) ? yield(:news_content) : yield with simply yield.

4 Changelog

• April 4, 2010: Fixed document to validate XHTML 1.0 Strict. Jaime Iniesta
• January 25, 2010: Rails 3.0 Update by Mikel Lindsaar
• December 27, 2008: Merge patch from Rodrigo Rosenfeld Rosas covering subtemplates
• December 27, 2008: Information on new rendering defaults by Mike Gunderloy
• November 9, 2008: Added partial collection counter by Mike Gunderloy
• November 1, 2008: Added :js option for render by Mike Gunderloy
• October 16, 2008: Ready for publication by Mike Gunderloy
• October 4, 2008: Additional info on partials (:object, :as, and :spacer_template) by Mike Gunderloy (not yet approved for publication)
• September 28, 2008: First draft by Mike Gunderloy (not yet approved for publication)

Feedback

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If you see any typos or factual errors you are confident to patch, please clone docrails and push the change yourself. That branch of Rails has public write access. Commits are still reviewed, but that happens after you've submitted your contribution. docrails is cross-merged with master periodically.

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If for whatever reason you spot something to fix but cannot patch it yourself, please open an issue.

And last but not least, any kind of discussion regarding Ruby on Rails documentation is very welcome in the rubyonrails-docs mailing list.