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 %>
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. Beginning with Rails 2, the standard extensions are .erb for ERB (HTML with embedded Ruby), and .builder for Builder (XML generator).
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.
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
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.
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
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
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 Rails 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 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 %>"
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
2.2.7 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 text 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:
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 and symbolic 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 for Controllers
You can override the default layout conventions in your controllers by using the layout declaration. For example:
class ProductsController < ApplicationController layout "inventory" #... end
With this declaration, all of the 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 layout declaration in your ApplicationController class:
class ApplicationController < ActionController::Base layout "main" #... end
With this declaration, all of the 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, such as a Proc, to determine the layout. For example, if you pass a Proc object, the block you give the Proc will be given the controller instance, so the layout can be determined based on the current request:
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 the :only and :except options. These options take either a method name, or an array of method names, corresponding 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
Layout declarations cascade downward in the hierarchy, and more specific layout declarations 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 false 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 to use and return instead of && return because && return will not work due to the operator precedence in the Ruby Language.
Note that the implicit render done by ActionController detects if render has been called, so 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_url
You can use redirect_to with any arguments that you could use with link_to or url_for. There’s also 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, a temporary redirect, when you call redirect_to. If you’d like to use a different status code, perhaps 301, a permanent redirect, you can use 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 think 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 will probably require. 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 application, this added latency might not be a problem, it is something to think about if response time is a concern. We can demonstrate one way to handle this with a contrived example:
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
This would detect that there are no books with the specified ID, populate the @books instance variable with all the books in the model, and then directly render the index.html.erb template, returning it to the browser with a flash alert message to tell 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
This 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 other 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 Tag Helpers
Asset tag helpers provide methods for generating HTML that link views to feeds, JavaScript, stylesheets, images, videos and audios. There are six asset tag helpers 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 auto_discovery_link_tag, javascript_include_tag, and stylesheet_link_tag, are most commonly used in the <head> section of a layout.
The asset tag helpers 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.
3.1.1 Linking to Feeds with the 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 the auto_discovery_link_tag:
- :rel specifies the rel value in the link. The default value is “alternate”.
- :type specifies an explicit MIME type. Rails will generate an appropriate MIME type automatically.
- :title specifies the title of the link. The default value is the upshifted :type value, for example, “ATOM” or “RSS”.
3.1.2 Linking to JavaScript Files with the javascript_include_tag
The javascript_include_tag helper returns an HTML script tag for each source provided.
If you are using Rails with the Asset Pipeline enabled, this helper will generate a link to /assets/javascripts/ rather than public/javascripts which was used in earlier versions of Rails. This link is then served by the Sprockets gem, which was introduced in Rails 3.1.
A JavaScript file within a Rails application or Rails engine goes in one of three locations: app/assets, lib/assets or vendor/assets. These locations are explained in detail in the Asset Organization section in the Asset Pipeline Guide
You can specify a full path relative to the document root, or a URL, if you prefer. For example, to link to a JavaScript file that is inside a directory called javascripts inside of one of app/assets, lib/assets or vendor/assets, you would do this:
<%= javascript_include_tag "main" %>
Rails will then output a script tag such as this:
<script src="/img/spacer.gif">
The request to this asset is then served by the Sprockets gem.
To include multiple files such as app/assets/javascripts/main.js and app/assets/javascripts/columns.js at the same time:
<%= javascript_include_tag "main", "columns" %>
To include app/assets/javascripts/main.js and app/assets/javascripts/photos/columns.js:
<%= javascript_include_tag "main", "/photos/columns" %>
To include example.com/main.js:
<%= javascript_include_tag "example.com/main.js" %>
If the application does not use the asset pipeline, the :defaults option loads jQuery by default:
<%= javascript_include_tag :defaults %>
Outputting script tags such as this:
<script src="/img/spacer.gif">
These two files for jQuery, jquery.js and jquery_ujs.js must be placed inside public/javascripts if the application doesn’t use the asset pipeline. These files can be downloaded from the jquery-rails repository on GitHub
If you are using the asset pipeline, this tag will render a script tag for an asset called defaults.js, which would not exist in your application unless you’ve explicitly defined it to be.
And you can in any case override the :defaults expansion in config/application.rb:
config.action_view.javascript_expansions[:defaults] = %w(foo.js bar.js)
You can also define new defaults:
config.action_view.javascript_expansions[:projects] = %w(projects.js tickets.js)
And use them by referencing them exactly like :defaults:
<%= javascript_include_tag :projects %>
When using :defaults, if an application.js file exists in public/javascripts it will be included as well at the end.
Also, if the asset pipeline is disabled, the :all expansion 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 the stylesheet_link_tag
The stylesheet_link_tag helper returns an HTML <link> tag for each source provided.
If you are using Rails with the “Asset Pipeline” enabled, this helper will generate a link to /assets/stylesheets/. This link is then processed by the Sprockets gem. A stylesheet file can be stored in one of three locations: app/assets, lib/assets or vendor/assets.