Available guides | Download | { :silence => :foo }
More at rubyonrails.org: Overview | Download | Deploy | Code | Screencasts | Documentation | Ecosystem | Community | Blog

Layouts and Rendering in Rails

This guide covers the basic layout features of Action Controller and Action View. By referring to this guide, you will be able to:

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 actions. For example, if you have this code in your BooksController class:

def show @book = Book.find(params[:id]) end

Rails will automatically render app/views/books/show.html.erb after running the method. In fact, if you have the default catch-all route in place (map.connect ':controller/:action/:id'), Rails will even render views that don’t have any code at all in the controller. For example, if you have the default route in place and a request comes in for /books/sale_list, Rails will render app/views/books/sale_list.html.erb in response.

The actual rendering is done by subclasses of ActionView::TemplateHandlers. This guide does not dig into that process, but it’s important to know that the file extension on your view controls the choice of template handler. In Rails 2, the standard extensions are .erb for ERB (HTML with embedded Ruby), .rjs for RJS (javascript with embedded ruby) and .builder for Builder (XML generator). You’ll also find .rhtml used for ERB templates and .rxml for Builder templates, but those extensions are now formally deprecated and will be removed from a future version of Rails.

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 behavior 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.

If you want to see the exact results of a call to render without needing to inspect it in a browser, you can call render_to_string. This method takes exactly the same options as render, but it returns a string instead of sending a response back to the browser.

2.2.1 Rendering Nothing

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

render :nothing => true

This will send an empty response to the browser (though it will include any status headers you set with the :status option, discussed below).

You should probably be using the head method, discussed later in this guide, instead of render :nothing. This provides additional flexibility and makes it explicit that you’re only generating HTTP headers.

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 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 end

To be explicit, you can use render with the :action option (though this is no longer necessary as of Rails 2.3):

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

Using render with :action is a frequent source of confusion for Rails newcomers. The specified action is used to determine which view to render, but Rails does not run any of the code for that action in the controller. Any instance variables that you require in the view must be set up in the current action before calling render.

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.

By default, the file is rendered without using the current layout. If you want Rails to put the file into the current layout, you need to add the :layout => true option.

If you’re running on Microsoft Windows, you should use the :file option to render a file, because Windows filenames do not have the same format as Unix filenames.

2.2.5 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 %>"

There is seldom any good reason to use this option. Mixing ERB into your controllers defeats the MVC orientation of Rails and will make it harder for other developers to follow the logic of your project. Use a separate erb view instead.

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
2.2.6 Using render with :update

You can also render javascript-based page updates inline using the :update option to render:

render :update do |page| page.replace_html 'warning', "Invalid options supplied" end

Placing javascript updates in your controller may seem to streamline small updates, but it defeats the MVC orientation of Rails and will make it harder for other developers to follow the logic of your project. We recommend using a separate rjs template instead, no matter how small the update.

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"

Rendering pure text is most useful when you’re responding to AJAX or web service requests that are expecting something other than proper HTML.

By default, if you use the :text option, the file is rendered without using the current layout. If you want Rails to put the text into the current layout, you need to add the :layout => true option

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

You don’t need to call to_json on the object that you want to render. If you use the :json option, render will automatically call to_json for you.

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

You don’t need to call to_xml on the object that you want to render. If you use the :xml option, render will automatically call to_xml for you.

2.2.10 Rendering Vanilla JavaScript

Rails can render vanilla JavaScript (as an alternative to using update with n .rjs file):

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 HTML 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 either numeric status codes or symbols for status codes. You can find its list of status codes in actionpack/lib/action_controller/status_codes.rb. You can also see there how Rails maps symbols to 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 ot /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:

class ProductsController < ApplicationController layout proc { |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:

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 show view – and throw an error. The solution is simple: make sure that you only have 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

Note that the implicit render done by ActionController detects if render has been called, and thus avoids this error. So this code will work with problems:

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 (permanent redirect) when you call redirect_to. If you’d like to use a different status code (perhaps 301, temporary 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.find(:all) end def show @book = Book.find(params[:id]) if @book.nil? render :action => "index" end end

With the code in this form, there will be 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.find(:all) end def show @book = Book.find(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.

2.4 Using head To Build Header-Only Responses

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

head :bad_request

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

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

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

I’ll discuss each of these in turn.

3.1 Asset Tags

Asset tags provide methods for generating HTML that links views to assets like images, javascript, stylesheets, and feeds. There are four types of include tag:

  • auto_discovery_link_tag
  • javascript_include_tag
  • stylesheet_link_tag
  • image_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.

The asset tags do not verify the existence of the assets at the specified locations; they simply assume that you know what you’re doing and generate the link.

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" %>

The defaults option loads the Prototype and Scriptaculous libraries:

<%= javascript_include_tag :defaults %>

The all option loads every javascript file in public/javascripts, starting with the Prototype and Scriptaculous libraries:

<%= javascript_include_tag :all %>

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.

The stylesheet_link_tag helper returns an HTML 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.cs:

<%= 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.cs:

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

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 <image> tag to the specified file. By default, files are loaded from public/images. If you don’t specify an extension, .png is assumed by default:

<%= image_tag "header" %>

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 %>

There are also three special options you can use with image_tag:

  • :alt specifies the alt text for the image (which defaults to the file name of the file, capitalized and with no extension)
  • :size specifies both width and height, in the format “{width}x{height}” (for example, “150×125”)
  • :mouseover sets an alternate image to be used when the onmouseover event is fired.

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 yield block in your layout. You only use content_for to insert content in named yields. 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 apart 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, and include the :partial option:

<%= render :partial => "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 :partial => "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 :partial => "shared/ad_banner" %> <h1>Products</h1> <p>Here are a few of our fine products:</p> ... <%= render :partial => "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.

For content that is shared among all pages in your application, you can use partials directly from layouts.

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).

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 => { :button_label => "Create zone", :zone => @zone } %>
  • edit.html.erb
<h1>Editing zone</h1> <%= error_messages_for :zone %> <%= render :partial => "form", :locals => { :button_label => "Update zone", :zone => @zone } %>
  • _form.html.erb
<% form_for(zone) do |f| %> <p> <b>Zone name</b><br /> <%= f.text_field :name %> </p> <p> <%= f.submit button_label %> </p> <% end %>

Although the same partial will be rendered into both views, the label on the submit button is controlled by a local variable passed into the partial.

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.

In previous versions of Rails, the default local variable would look for an instance variable with the same name as the partial in the parent. This behavior is deprecated in Rails 2.2 and will be removed in a future version.

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

<%= render :partial => @customer %>

Assuming that the @customer instance variable contains an instance of the Customer model, this will use _customer.html.erb to render it.

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. 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.

Rails also makes a counter variable available within a partial called by the collection, named after the member of the collection followed by _counter. For example, if you’re rendering @products, within the partial you can refer to product_counter to tell you how many times the partial has been rendered.

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

<%= render :partial => "product", :collection => @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.

There’s also a shorthand syntax available for rendering collections. For example, if @products is a collection of products, you can render the collection this way:

  • index.html.erb
<h1>Products</h1> <%= render :partial => @products %>
  • _product.html.erb
<p>Product Name: <%= product.name %></p>

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 :partial => [customer1, employee1, customer2, employee2] %>
  • _customer.html.erb
<p>Name: <%= customer.name %></p>
  • _employee.html.erb
<p>Name: <%= employee.name %></p>

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

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 follow 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"><%= yield(:content) or 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> <%= yield(:news_content) or yield %> <% end -%> <% render :file => '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 :file => 'layouts/news' to base a new layout on the News layout. If one is sure she will not subtemplate the News layout, she can ommit the yield(:news_content) or part.

4 Changelog

Lighthouse ticket

  • 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)