Layouts and Rendering in Rails ============================== This guide covers the basic layout features of Action Controller and Action View. After reading this guide, you will know: * How to use the various rendering methods built into Rails. * How to create layouts with multiple content sections. * How to use partials to DRY up your views. * How to use nested layouts (sub-templates). -------------------------------------------------------------------------------- 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. 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 ### 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: ```ruby class BooksController < ApplicationController end ``` And the following in your routes file: ```ruby resources :books ``` And you have a view file `app/views/books/index.html.erb`: ```html+erb

Books are coming soon!

``` 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`: ```ruby 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: ```html+erb

Listing Books

<% @books.each do |book| %> <% end %>
Title Summary
<%= book.title %> <%= book.content %> <%= link_to "Show", book %> <%= link_to "Edit", edit_book_path(book) %> <%= link_to "Remove", book, method: :delete, data: { confirm: "Are you sure?" } %>

<%= link_to "New book", new_book_path %> ``` NOTE: 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). ### 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. TIP: 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. #### Rendering Nothing Perhaps the simplest thing you can do with `render` is to render nothing at all: ```ruby render nothing: true ``` If you look at the response for this using cURL, you will see the following: ```bash $ 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. TIP: 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. #### Rendering an Action's View If you want to render the view that corresponds to a different template within the same controller, you can use `render` with the name of the view: ```ruby def update @book = Book.find(params[:id]) if @book.update(params[:book]) redirect_to(@book) else render "edit" end end ``` If the call to `update` 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: ```ruby def update @book = Book.find(params[:id]) if @book.update(params[:book]) redirect_to(@book) else render :edit end end ``` #### 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: ```ruby 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): ```ruby render template: "products/show" ``` #### 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): ```ruby 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): ```ruby 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. NOTE: 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. TIP: 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. #### 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: ```ruby 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. #### 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: ```ruby render inline: "<% products.each do |p| %>

<%= p.name %>

<% end %>" ``` WARNING: 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: ```ruby render inline: "xml.p {'Horrid coding practice!'}", type: :builder ``` #### Rendering Text You can send plain text - with no markup at all - back to the browser by using the `:text` option to `render`: ```ruby render text: "OK" ``` TIP: Rendering pure text is most useful when you're responding to Ajax or web service requests that are expecting something other than proper HTML. NOTE: 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. #### 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: ```ruby render json: @product ``` TIP: 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. #### Rendering XML Rails also has built-in support for converting objects to XML and rendering that XML back to the caller: ```ruby render xml: @product ``` TIP: 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. #### Rendering Vanilla JavaScript Rails can render vanilla JavaScript: ```ruby render js: "alert('Hello Rails');" ``` This will send the supplied string to the browser with a MIME type of `text/javascript`. #### Options for `render` Calls to the `render` method generally accept four options: * `:content_type` * `:layout` * `:status` * `:location` ##### 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: ```ruby render file: filename, content_type: "application/rss" ``` ##### 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: ```ruby render layout: "special_layout" ``` You can also tell Rails to render with no layout at all: ```ruby render layout: false ``` ##### 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: ```ruby render status: 500 render status: :forbidden ``` Rails understands both numeric and symbolic status codes. ##### The `:location` Option You can use the `:location` option to set the HTTP `Location` header: ```ruby render xml: photo, location: photo_url(photo) ``` #### 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. ##### Specifying Layouts for Controllers You can override the default layout conventions in your controllers by using the `layout` declaration. For example: ```ruby class ProductsController < ApplicationController layout "inventory" #... end ``` With this declaration, all of the views rendered by the products controller will use `app/views/layouts/inventory.html.erb` as their layout. To assign a specific layout for the entire application, use a `layout` declaration in your `ApplicationController` class: ```ruby 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. ##### Choosing Layouts at Runtime You can use a symbol to defer the choice of layout until a request is processed: ```ruby 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: ```ruby class ProductsController < ApplicationController layout Proc.new { |controller| controller.request.xhr? ? "popup" : "application" } end ``` ##### 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: ```ruby 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. ##### Layout Inheritance Layout declarations cascade downward in the hierarchy, and more specific layout declarations always override more general ones. For example: * `application_controller.rb` ```ruby class ApplicationController < ActionController::Base layout "main" end ``` * `posts_controller.rb` ```ruby class PostsController < ApplicationController end ``` * `special_posts_controller.rb` ```ruby class SpecialPostsController < PostsController layout "special" end ``` * `old_posts_controller.rb` ```ruby 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 #### 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: ```ruby 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: ```ruby 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: ```ruby 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. ### 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: ```ruby 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: ```ruby redirect_to :back ``` #### 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: ```ruby redirect_to photos_path, status: 301 ``` Just like the `:status` option for `render`, `:status` for `redirect_to` accepts both numeric and symbolic header designations. #### 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: ```ruby 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: ```ruby 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: ```ruby def index @books = Book.all end def show @book = Book.find_by_id(params[:id]) if @book.nil? @books = Book.all flash[:alert] = "Your book was not found" render "index" 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. ### 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: ```ruby 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: ```ruby 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 ``` 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 ### 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 `` section of a layout. WARNING: 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. #### 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 presence 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: ```erb <%= 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 uppercased `:type` value, for example, "ATOM" or "RSS". #### 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](asset_pipeline.html) 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 asset pipeline. 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](asset_pipeline.html#asset-organization) 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: ```erb <%= javascript_include_tag "main" %> ``` Rails will then output a `script` tag such as this: ```html ``` 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: ```erb <%= javascript_include_tag "main", "columns" %> ``` To include `app/assets/javascripts/main.js` and `app/assets/javascripts/photos/columns.js`: ```erb <%= javascript_include_tag "main", "/photos/columns" %> ``` To include `http://example.com/main.js`: ```erb <%= javascript_include_tag "http://example.com/main.js" %> ``` #### Linking to CSS Files with the `stylesheet_link_tag` The `stylesheet_link_tag` helper returns an HTML `` 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`. You can specify a full path relative to the document root, or a URL. For example, to link to a stylesheet file that is inside a directory called `stylesheets` inside of one of `app/assets`, `lib/assets` or `vendor/assets`, you would do this: ```erb <%= stylesheet_link_tag "main" %> ``` To include `app/assets/stylesheets/main.css` and `app/assets/stylesheets/columns.css`: ```erb <%= stylesheet_link_tag "main", "columns" %> ``` To include `app/assets/stylesheets/main.css` and `app/assets/stylesheets/photos/columns.css`: ```erb <%= stylesheet_link_tag "main", "photos/columns" %> ``` To include `http://example.com/main.css`: ```erb <%= stylesheet_link_tag "http://example.com/main.css" %> ``` By default, the `stylesheet_link_tag` creates links with `media="screen" rel="stylesheet"`. You can override any of these defaults by specifying an appropriate option (`:media`, `:rel`): ```erb <%= stylesheet_link_tag "main_print", media: "print" %> ``` #### Linking to Images with the `image_tag` The `image_tag` helper builds an HTML `` tag to the specified file. By default, files are loaded from `public/images`. WARNING: Note that you must specify the extension of the image. ```erb <%= image_tag "header.png" %> ``` You can supply a path to the image if you like: ```erb <%= image_tag "icons/delete.gif" %> ``` You can supply a hash of additional HTML options: ```erb <%= image_tag "icons/delete.gif", {height: 45} %> ``` 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: ```erb <%= image_tag "home.gif" %> <%= image_tag "home.gif", alt: "Home" %> ``` You can also specify a special size tag, in the format "{width}x{height}": ```erb <%= 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`: ```erb <%= image_tag "home.gif", alt: "Go Home", id: "HomeImage", class: "nav_bar" %> ``` #### Linking to Videos with the `video_tag` The `video_tag` helper builds an HTML 5 `