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<h2>Layouts and Rendering in Rails</h2><p>This guide covers the basic layout features of Action Controller and Action View.</p><p>After reading this guide, you will know:</p>
<ul>
<li>How to use the various rendering methods built into Rails.</li>
<li>How to create layouts with multiple content sections.</li>
<li>How to use partials to DRY up your views.</li>
<li>How to use nested layouts (sub-templates).</li>
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<h3 class="chapter"><img src="images/chapters_icon.gif" alt="" />Chapters</h3>
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<li><a href="#overview-how-the-pieces-fit-together">Overview: How the Pieces Fit Together</a></li>
<li>
<a href="#creating-responses">Creating Responses</a>
<ul>
<li><a href="#rendering-by-default-convention-over-configuration-in-action">Rendering by Default: Convention Over Configuration in Action</a></li>
<li><a href="#using-render">Using <code>render</code></a></li>
<li><a href="#using-redirect-to">Using <code>redirect_to</code></a></li>
<li><a href="#using-head-to-build-header-only-responses">Using <code>head</code> To Build Header-Only Responses</a></li>
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<li><a href="#using-the-content-for-method">Using the <code>content_for</code> Method</a></li>
<li><a href="#using-partials">Using Partials</a></li>
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<h3 id="overview-how-the-pieces-fit-together"><a class="anchorlink" href="#overview-how-the-pieces-fit-together">1 Overview: How the Pieces Fit Together</a></h3><p>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.</p><p>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.</p><h3 id="creating-responses"><a class="anchorlink" href="#creating-responses">2 Creating Responses</a></h3><p>From the controller's point of view, there are three ways to create an HTTP response:</p>
<ul>
<li>Call <code>render</code> to create a full response to send back to the browser</li>
<li>Call <code>redirect_to</code> to send an HTTP redirect status code to the browser</li>
<li>Call <code>head</code> to create a response consisting solely of HTTP headers to send back to the browser</li>
</ul>
<h4 id="rendering-by-default-convention-over-configuration-in-action"><a class="anchorlink" href="#rendering-by-default-convention-over-configuration-in-action">2.1 Rendering by Default: Convention Over Configuration in Action</a></h4><p>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 <code>BooksController</code> class:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class BooksController < ApplicationController
end
</pre>
</div>
<p>And the following in your routes file:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
resources :books
</pre>
</div>
<p>And you have a view file <code>app/views/books/index.html.erb</code>:</p><div class="code_container">
<pre class="brush: ruby; html-script: true; gutter: false; toolbar: false">
<h1>Books are coming soon!</h1>
</pre>
</div>
<p>Rails will automatically render <code>app/views/books/index.html.erb</code> when you navigate to <code>/books</code> and you will see "Books are coming soon!" on your screen.</p><p>However a coming soon screen is only minimally useful, so you will soon create your <code>Book</code> model and add the index action to <code>BooksController</code>:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class BooksController < ApplicationController
def index
@books = Book.all
end
end
</pre>
</div>
<p>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 <code>action_name.html.erb</code> template in the controller's view path and render it. So in this case, Rails will render the <code>app/views/books/index.html.erb</code> file.</p><p>If we want to display the properties of all the books in our view, we can do so with an ERB template like this:</p><div class="code_container">
<pre class="brush: ruby; html-script: true; gutter: false; toolbar: false">
<h1>Listing Books</h1>
<table>
<thead>
<tr>
<th>Title</th>
<th>Content</th>
<th colspan="3"></th>
</tr>
</thead>
<tbody>
<% @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 "Destroy", book, method: :delete, data: { confirm: "Are you sure?" } %></td>
</tr>
<% end %>
</tbody>
</table>
<br>
<%= link_to "New book", new_book_path %>
</pre>
</div>
<div class="note"><p>The actual rendering is done by nested classes of the module <a href="http://edgeapi.rubyonrails.org/classes/ActionView/Template/Handlers.html"><code>ActionView::Template::Handlers</code></a>. 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.</p></div><h4 id="using-render"><a class="anchorlink" href="#using-render">2.2 Using <code>render</code></a></h4><p>In most cases, the <code>ActionController::Base#render</code> 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 <code>render</code>. 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.</p><div class="info"><p>If you want to see the exact results of a call to <code>render</code> without needing to inspect it in a browser, you can call <code>render_to_string</code>. This method takes exactly the same options as <code>render</code>, but it returns a string instead of sending a response back to the browser.</p></div><h5 id="rendering-an-action-s-view"><a class="anchorlink" href="#rendering-an-action-s-view">2.2.1 Rendering an Action's View</a></h5><p>If you want to render the view that corresponds to a different template within the same controller, you can use <code>render</code> with the name of the view:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def update
@book = Book.find(params[:id])
if @book.update(book_params)
redirect_to(@book)
else
render "edit"
end
end
</pre>
</div>
<p>If the call to <code>update</code> fails, calling the <code>update</code> action in this controller will render the <code>edit.html.erb</code> template belonging to the same controller.</p><p>If you prefer, you can use a symbol instead of a string to specify the action to render:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def update
@book = Book.find(params[:id])
if @book.update(book_params)
redirect_to(@book)
else
render :edit
end
end
</pre>
</div>
<h5 id="rendering-an-action-s-template-from-another-controller"><a class="anchorlink" href="#rendering-an-action-s-template-from-another-controller">2.2.2 Rendering an Action's Template from Another Controller</a></h5><p>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 <code>render</code>, which accepts the full path (relative to <code>app/views</code>) of the template to render. For example, if you're running code in an <code>AdminProductsController</code> that lives in <code>app/controllers/admin</code>, you can render the results of an action to a template in <code>app/views/products</code> this way:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render "products/show"
</pre>
</div>
<p>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 <code>:template</code> option (which was required on Rails 2.2 and earlier):</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render template: "products/show"
</pre>
</div>
<h5 id="rendering-an-arbitrary-file"><a class="anchorlink" href="#rendering-an-arbitrary-file">2.2.3 Rendering an Arbitrary File</a></h5><p>The <code>render</code> method can also use a view that's entirely outside of your application:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render file: "/u/apps/warehouse_app/current/app/views/products/show"
</pre>
</div>
<p>The <code>:file</code> 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.</p><div class="note"><p>Using the <code>:file</code> option in combination with users input can lead to security problems
since an attacker could use this action to access security sensitive files in your file system.</p></div><div class="note"><p>By default, the file is rendered using the current layout.</p></div><div class="info"><p>If you're running Rails on Microsoft Windows, you should use the <code>:file</code> option to
render a file, because Windows filenames do not have the same format as Unix filenames.</p></div><h5 id="wrapping-it-up"><a class="anchorlink" href="#wrapping-it-up">2.2.4 Wrapping it up</a></h5><p>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.</p><p>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 <code>edit.html.erb</code> template in the <code>views/books</code> directory:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
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"
</pre>
</div>
<p>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.</p><h5 id="using-render-with-inline"><a class="anchorlink" href="#using-render-with-inline">2.2.5 Using <code>render</code> with <code>:inline</code></a></h5><p>The <code>render</code> method can do without a view completely, if you're willing to use the <code>:inline</code> option to supply ERB as part of the method call. This is perfectly valid:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render inline: "<% products.each do |p| %><p><%= p.name %></p><% end %>"
</pre>
</div>
<div class="warning"><p>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.</p></div><p>By default, inline rendering uses ERB. You can force it to use Builder instead with the <code>:type</code> option:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render inline: "xml.p {'Horrid coding practice!'}", type: :builder
</pre>
</div>
<h5 id="rendering-text"><a class="anchorlink" href="#rendering-text">2.2.6 Rendering Text</a></h5><p>You can send plain text - with no markup at all - back to the browser by using
the <code>:plain</code> option to <code>render</code>:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render plain: "OK"
</pre>
</div>
<div class="info"><p>Rendering pure text is most useful when you're responding to Ajax or web
service requests that are expecting something other than proper HTML.</p></div><div class="note"><p>By default, if you use the <code>:plain</code> 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 <code>layout: true</code> option and use the <code>.text.erb</code>
extension for the layout file.</p></div><h5 id="rendering-html"><a class="anchorlink" href="#rendering-html">2.2.7 Rendering HTML</a></h5><p>You can send an HTML string back to the browser by using the <code>:html</code> option to
<code>render</code>:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render html: helpers.tag.strong('Not Found')
</pre>
</div>
<div class="info"><p>This is useful when you're rendering a small snippet of HTML code.
However, you might want to consider moving it to a template file if the markup
is complex.</p></div><div class="note"><p>When using <code>html:</code> option, HTML entities will be escaped if the string is not composed with <code>html_safe</code>-aware APIs.</p></div><h5 id="rendering-json"><a class="anchorlink" href="#rendering-json">2.2.8 Rendering JSON</a></h5><p>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:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render json: @product
</pre>
</div>
<div class="info"><p>You don't need to call <code>to_json</code> on the object that you want to render. If you use the <code>:json</code> option, <code>render</code> will automatically call <code>to_json</code> for you.</p></div><h5 id="rendering-xml"><a class="anchorlink" href="#rendering-xml">2.2.9 Rendering XML</a></h5><p>Rails also has built-in support for converting objects to XML and rendering that XML back to the caller:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render xml: @product
</pre>
</div>
<div class="info"><p>You don't need to call <code>to_xml</code> on the object that you want to render. If you use the <code>:xml</code> option, <code>render</code> will automatically call <code>to_xml</code> for you.</p></div><h5 id="rendering-vanilla-javascript"><a class="anchorlink" href="#rendering-vanilla-javascript">2.2.10 Rendering Vanilla JavaScript</a></h5><p>Rails can render vanilla JavaScript:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render js: "alert('Hello Rails');"
</pre>
</div>
<p>This will send the supplied string to the browser with a MIME type of <code>text/javascript</code>.</p><h5 id="rendering-raw-body"><a class="anchorlink" href="#rendering-raw-body">2.2.11 Rendering raw body</a></h5><p>You can send a raw content back to the browser, without setting any content
type, by using the <code>:body</code> option to <code>render</code>:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render body: "raw"
</pre>
</div>
<div class="info"><p>This option should be used only if you don't care about the content type of
the response. Using <code>:plain</code> or <code>:html</code> might be more appropriate most of the
time.</p></div><div class="note"><p>Unless overridden, your response returned from this render option will be
<code>text/plain</code>, as that is the default content type of Action Dispatch response.</p></div><h5 id="options-for-render"><a class="anchorlink" href="#options-for-render">2.2.12 Options for <code>render</code></a></h5><p>Calls to the <code>render</code> method generally accept five options:</p>
<ul>
<li><code>:content_type</code></li>
<li><code>:layout</code></li>
<li><code>:location</code></li>
<li><code>:status</code></li>
<li><code>:formats</code></li>
</ul>
<h6 id="the-content-type-option"><a class="anchorlink" href="#the-content-type-option">2.2.12.1 The <code>:content_type</code> Option</a></h6><p>By default, Rails will serve the results of a rendering operation with the MIME content-type of <code>text/html</code> (or <code>application/json</code> if you use the <code>:json</code> option, or <code>application/xml</code> for the <code>:xml</code> option.). There are times when you might like to change this, and you can do so by setting the <code>:content_type</code> option:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render file: filename, content_type: "application/rss"
</pre>
</div>
<h6 id="the-layout-option"><a class="anchorlink" href="#the-layout-option">2.2.12.2 The <code>:layout</code> Option</a></h6><p>With most of the options to <code>render</code>, 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.</p><p>You can use the <code>:layout</code> option to tell Rails to use a specific file as the layout for the current action:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render layout: "special_layout"
</pre>
</div>
<p>You can also tell Rails to render with no layout at all:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render layout: false
</pre>
</div>
<h6 id="the-location-option"><a class="anchorlink" href="#the-location-option">2.2.12.3 The <code>:location</code> Option</a></h6><p>You can use the <code>:location</code> option to set the HTTP <code>Location</code> header:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render xml: photo, location: photo_url(photo)
</pre>
</div>
<h6 id="the-status-option"><a class="anchorlink" href="#the-status-option">2.2.12.4 The <code>:status</code> Option</a></h6><p>Rails will automatically generate a response with the correct HTTP status code (in most cases, this is <code>200 OK</code>). You can use the <code>:status</code> option to change this:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render status: 500
render status: :forbidden
</pre>
</div>
<p>Rails understands both numeric status codes and the corresponding symbols shown below.</p>
<table>
<thead>
<tr>
<th>Response Class</th>
<th>HTTP Status Code</th>
<th>Symbol</th>
</tr>
</thead>
<tbody>
<tr>
<td><strong>Informational</strong></td>
<td>100</td>
<td>:continue</td>
</tr>
<tr>
<td></td>
<td>101</td>
<td>:switching_protocols</td>
</tr>
<tr>
<td></td>
<td>102</td>
<td>:processing</td>
</tr>
<tr>
<td><strong>Success</strong></td>
<td>200</td>
<td>:ok</td>
</tr>
<tr>
<td></td>
<td>201</td>
<td>:created</td>
</tr>
<tr>
<td></td>
<td>202</td>
<td>:accepted</td>
</tr>
<tr>
<td></td>
<td>203</td>
<td>:non_authoritative_information</td>
</tr>
<tr>
<td></td>
<td>204</td>
<td>:no_content</td>
</tr>
<tr>
<td></td>
<td>205</td>
<td>:reset_content</td>
</tr>
<tr>
<td></td>
<td>206</td>
<td>:partial_content</td>
</tr>
<tr>
<td></td>
<td>207</td>
<td>:multi_status</td>
</tr>
<tr>
<td></td>
<td>208</td>
<td>:already_reported</td>
</tr>
<tr>
<td></td>
<td>226</td>
<td>:im_used</td>
</tr>
<tr>
<td><strong>Redirection</strong></td>
<td>300</td>
<td>:multiple_choices</td>
</tr>
<tr>
<td></td>
<td>301</td>
<td>:moved_permanently</td>
</tr>
<tr>
<td></td>
<td>302</td>
<td>:found</td>
</tr>
<tr>
<td></td>
<td>303</td>
<td>:see_other</td>
</tr>
<tr>
<td></td>
<td>304</td>
<td>:not_modified</td>
</tr>
<tr>
<td></td>
<td>305</td>
<td>:use_proxy</td>
</tr>
<tr>
<td></td>
<td>307</td>
<td>:temporary_redirect</td>
</tr>
<tr>
<td></td>
<td>308</td>
<td>:permanent_redirect</td>
</tr>
<tr>
<td><strong>Client Error</strong></td>
<td>400</td>
<td>:bad_request</td>
</tr>
<tr>
<td></td>
<td>401</td>
<td>:unauthorized</td>
</tr>
<tr>
<td></td>
<td>402</td>
<td>:payment_required</td>
</tr>
<tr>
<td></td>
<td>403</td>
<td>:forbidden</td>
</tr>
<tr>
<td></td>
<td>404</td>
<td>:not_found</td>
</tr>
<tr>
<td></td>
<td>405</td>
<td>:method_not_allowed</td>
</tr>
<tr>
<td></td>
<td>406</td>
<td>:not_acceptable</td>
</tr>
<tr>
<td></td>
<td>407</td>
<td>:proxy_authentication_required</td>
</tr>
<tr>
<td></td>
<td>408</td>
<td>:request_timeout</td>
</tr>
<tr>
<td></td>
<td>409</td>
<td>:conflict</td>
</tr>
<tr>
<td></td>
<td>410</td>
<td>:gone</td>
</tr>
<tr>
<td></td>
<td>411</td>
<td>:length_required</td>
</tr>
<tr>
<td></td>
<td>412</td>
<td>:precondition_failed</td>
</tr>
<tr>
<td></td>
<td>413</td>
<td>:payload_too_large</td>
</tr>
<tr>
<td></td>
<td>414</td>
<td>:uri_too_long</td>
</tr>
<tr>
<td></td>
<td>415</td>
<td>:unsupported_media_type</td>
</tr>
<tr>
<td></td>
<td>416</td>
<td>:range_not_satisfiable</td>
</tr>
<tr>
<td></td>
<td>417</td>
<td>:expectation_failed</td>
</tr>
<tr>
<td></td>
<td>421</td>
<td>:misdirected_request</td>
</tr>
<tr>
<td></td>
<td>422</td>
<td>:unprocessable_entity</td>
</tr>
<tr>
<td></td>
<td>423</td>
<td>:locked</td>
</tr>
<tr>
<td></td>
<td>424</td>
<td>:failed_dependency</td>
</tr>
<tr>
<td></td>
<td>426</td>
<td>:upgrade_required</td>
</tr>
<tr>
<td></td>
<td>428</td>
<td>:precondition_required</td>
</tr>
<tr>
<td></td>
<td>429</td>
<td>:too_many_requests</td>
</tr>
<tr>
<td></td>
<td>431</td>
<td>:request_header_fields_too_large</td>
</tr>
<tr>
<td></td>
<td>451</td>
<td>:unavailable_for_legal_reasons</td>
</tr>
<tr>
<td><strong>Server Error</strong></td>
<td>500</td>
<td>:internal_server_error</td>
</tr>
<tr>
<td></td>
<td>501</td>
<td>:not_implemented</td>
</tr>
<tr>
<td></td>
<td>502</td>
<td>:bad_gateway</td>
</tr>
<tr>
<td></td>
<td>503</td>
<td>:service_unavailable</td>
</tr>
<tr>
<td></td>
<td>504</td>
<td>:gateway_timeout</td>
</tr>
<tr>
<td></td>
<td>505</td>
<td>:http_version_not_supported</td>
</tr>
<tr>
<td></td>
<td>506</td>
<td>:variant_also_negotiates</td>
</tr>
<tr>
<td></td>
<td>507</td>
<td>:insufficient_storage</td>
</tr>
<tr>
<td></td>
<td>508</td>
<td>:loop_detected</td>
</tr>
<tr>
<td></td>
<td>510</td>
<td>:not_extended</td>
</tr>
<tr>
<td></td>
<td>511</td>
<td>:network_authentication_required</td>
</tr>
</tbody>
</table>
<div class="note"><p>If you try to render content along with a non-content status code
(100-199, 204, 205, or 304), it will be dropped from the response.</p></div><h6 id="the-formats-option"><a class="anchorlink" href="#the-formats-option">2.2.12.5 The <code>:formats</code> Option</a></h6><p>Rails uses the format specified in the request (or <code>:html</code> by default). You can
change this passing the <code>:formats</code> option with a symbol or an array:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
render formats: :xml
render formats: [:json, :xml]
</pre>
</div>
<p>If a template with the specified format does not exist an <code>ActionView::MissingTemplate</code> error is raised.</p><h5 id="finding-layouts"><a class="anchorlink" href="#finding-layouts">2.2.13 Finding Layouts</a></h5><p>To find the current layout, Rails first looks for a file in <code>app/views/layouts</code> with the same base name as the controller. For example, rendering actions from the <code>PhotosController</code> class will use <code>app/views/layouts/photos.html.erb</code> (or <code>app/views/layouts/photos.builder</code>). If there is no such controller-specific layout, Rails will use <code>app/views/layouts/application.html.erb</code> or <code>app/views/layouts/application.builder</code>. If there is no <code>.erb</code> layout, Rails will use a <code>.builder</code> layout if one exists. Rails also provides several ways to more precisely assign specific layouts to individual controllers and actions.</p><h6 id="specifying-layouts-for-controllers"><a class="anchorlink" href="#specifying-layouts-for-controllers">2.2.13.1 Specifying Layouts for Controllers</a></h6><p>You can override the default layout conventions in your controllers by using the <code>layout</code> declaration. For example:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ProductsController < ApplicationController
layout "inventory"
#...
end
</pre>
</div>
<p>With this declaration, all of the views rendered by the <code>ProductsController</code> will use <code>app/views/layouts/inventory.html.erb</code> as their layout.</p><p>To assign a specific layout for the entire application, use a <code>layout</code> declaration in your <code>ApplicationController</code> class:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ApplicationController < ActionController::Base
layout "main"
#...
end
</pre>
</div>
<p>With this declaration, all of the views in the entire application will use <code>app/views/layouts/main.html.erb</code> for their layout.</p><h6 id="choosing-layouts-at-runtime"><a class="anchorlink" href="#choosing-layouts-at-runtime">2.2.13.2 Choosing Layouts at Runtime</a></h6><p>You can use a symbol to defer the choice of layout until a request is processed:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
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
</pre>
</div>
<p>Now, if the current user is a special user, they'll get a special layout when viewing a product.</p><p>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 <code>controller</code> instance, so the layout can be determined based on the current request:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ProductsController < ApplicationController
layout Proc.new { |controller| controller.request.xhr? ? "popup" : "application" }
end
</pre>
</div>
<h6 id="conditional-layouts"><a class="anchorlink" href="#conditional-layouts">2.2.13.3 Conditional Layouts</a></h6><p>Layouts specified at the controller level support the <code>:only</code> and <code>:except</code> options. These options take either a method name, or an array of method names, corresponding to method names within the controller:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ProductsController < ApplicationController
layout "product", except: [:index, :rss]
end
</pre>
</div>
<p>With this declaration, the <code>product</code> layout would be used for everything but the <code>rss</code> and <code>index</code> methods.</p><h6 id="layout-inheritance"><a class="anchorlink" href="#layout-inheritance">2.2.13.4 Layout Inheritance</a></h6><p>Layout declarations cascade downward in the hierarchy, and more specific layout declarations always override more general ones. For example:</p>
<ul>
<li>
<p><code>application_controller.rb</code></p>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ApplicationController < ActionController::Base
layout "main"
end
</pre>
</div>
</li>
<li>
<p><code>articles_controller.rb</code></p>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ArticlesController < ApplicationController
end
</pre>
</div>
</li>
<li>
<p><code>special_articles_controller.rb</code></p>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class SpecialArticlesController < ArticlesController
layout "special"
end
</pre>
</div>
</li>
<li>
<p><code>old_articles_controller.rb</code></p>
<div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class OldArticlesController < SpecialArticlesController
layout false
def show
@article = Article.find(params[:id])
end
def index
@old_articles = Article.older
render layout: "old"
end
# ...
end
</pre>
</div>
</li>
</ul>
<p>In this application:</p>
<ul>
<li>In general, views will be rendered in the <code>main</code> layout</li>
<li>
<code>ArticlesController#index</code> will use the <code>main</code> layout</li>
<li>
<code>SpecialArticlesController#index</code> will use the <code>special</code> layout</li>
<li>
<code>OldArticlesController#show</code> will use no layout at all</li>
<li>
<code>OldArticlesController#index</code> will use the <code>old</code> layout</li>
</ul>
<h6 id="template-inheritance"><a class="anchorlink" href="#template-inheritance">2.2.13.5 Template Inheritance</a></h6><p>Similar to the Layout Inheritance logic, if a template or partial is not found in the conventional path, the controller will look for a template or partial to render in its inheritance chain. For example:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
# in app/controllers/application_controller
class ApplicationController < ActionController::Base
end
# in app/controllers/admin_controller
class AdminController < ApplicationController
end
# in app/controllers/admin/products_controller
class Admin::ProductsController < AdminController
def index
end
end
</pre>
</div>
<p>The lookup order for an <code>admin/products#index</code> action will be:</p>
<ul>
<li><code>app/views/admin/products/</code></li>
<li><code>app/views/admin/</code></li>
<li><code>app/views/application/</code></li>
</ul>
<p>This makes <code>app/views/application/</code> a great place for your shared partials, which can then be rendered in your ERB as such:</p><div class="code_container">
<pre class="brush: ruby; html-script: true; gutter: false; toolbar: false">
<%# app/views/admin/products/index.html.erb %>
<%= render @products || "empty_list" %>
<%# app/views/application/_empty_list.html.erb %>
There are no items in this list <em>yet</em>.
</pre>
</div>
<h5 id="avoiding-double-render-errors"><a class="anchorlink" href="#avoiding-double-render-errors">2.2.14 Avoiding Double Render Errors</a></h5><p>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 <code>render</code> works.</p><p>For example, here's some code that will trigger this error:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def show
@book = Book.find(params[:id])
if @book.special?
render action: "special_show"
end
render action: "regular_show"
end
</pre>
</div>
<p>If <code>@book.special?</code> evaluates to <code>true</code>, Rails will start the rendering process to dump the <code>@book</code> variable into the <code>special_show</code> view. But this will <em>not</em> stop the rest of the code in the <code>show</code> action from running, and when Rails hits the end of the action, it will start to render the <code>regular_show</code> view - and throw an error. The solution is simple: make sure that you have only one call to <code>render</code> or <code>redirect</code> in a single code path. One thing that can help is <code>and return</code>. Here's a patched version of the method:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def show
@book = Book.find(params[:id])
if @book.special?
render action: "special_show" and return
end
render action: "regular_show"
end
</pre>
</div>
<p>Make sure to use <code>and return</code> instead of <code>&& return</code> because <code>&& return</code> will not work due to the operator precedence in the Ruby Language.</p><p>Note that the implicit render done by ActionController detects if <code>render</code> has been called, so the following will work without errors:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def show
@book = Book.find(params[:id])
if @book.special?
render action: "special_show"
end
end
</pre>
</div>
<p>This will render a book with <code>special?</code> set with the <code>special_show</code> template, while other books will render with the default <code>show</code> template.</p><h4 id="using-redirect-to"><a class="anchorlink" href="#using-redirect-to">2.3 Using <code>redirect_to</code></a></h4><p>Another way to handle returning responses to an HTTP request is with <code>redirect_to</code>. As you've seen, <code>render</code> tells Rails which view (or other asset) to use in constructing a response. The <code>redirect_to</code> 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:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
redirect_to photos_url
</pre>
</div>
<p>You can use <code>redirect_back</code> to return the user to the page they just came from.
This location is pulled from the <code>HTTP_REFERER</code> header which is not guaranteed
to be set by the browser, so you must provide the <code>fallback_location</code>
to use in this case.</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
redirect_back(fallback_location: root_path)
</pre>
</div>
<div class="note"><p><code>redirect_to</code> and <code>redirect_back</code> do not halt and return immediately from method execution, but simply set HTTP responses. Statements occurring after them in a method will be executed. You can halt by an explicit <code>return</code> or some other halting mechanism, if needed.</p></div><h5 id="getting-a-different-redirect-status-code"><a class="anchorlink" href="#getting-a-different-redirect-status-code">2.3.1 Getting a Different Redirect Status Code</a></h5><p>Rails uses HTTP status code 302, a temporary redirect, when you call <code>redirect_to</code>. If you'd like to use a different status code, perhaps 301, a permanent redirect, you can use the <code>:status</code> option:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
redirect_to photos_path, status: 301
</pre>
</div>
<p>Just like the <code>:status</code> option for <code>render</code>, <code>:status</code> for <code>redirect_to</code> accepts both numeric and symbolic header designations.</p><h5 id="the-difference-between-render-and-redirect-to"><a class="anchorlink" href="#the-difference-between-render-and-redirect-to">2.3.2 The Difference Between <code>render</code> and <code>redirect_to</code></a></h5><p>Sometimes inexperienced developers think of <code>redirect_to</code> as a sort of <code>goto</code> command, moving execution from one place to another in your Rails code. This is <em>not</em> 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.</p><p>Consider these actions to see the difference:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def index
@books = Book.all
end
def show
@book = Book.find_by(id: params[:id])
if @book.nil?
render action: "index"
end
end
</pre>
</div>
<p>With the code in this form, there will likely be a problem if the <code>@book</code> variable is <code>nil</code>. Remember, a <code>render :action</code> doesn't run any code in the target action, so nothing will set up the <code>@books</code> variable that the <code>index</code> view will probably require. One way to fix this is to redirect instead of rendering:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def index
@books = Book.all
end
def show
@book = Book.find_by(id: params[:id])
if @book.nil?
redirect_to action: :index
end
end
</pre>
</div>
<p>With this code, the browser will make a fresh request for the index page, the code in the <code>index</code> method will run, and all will be well.</p><p>The only downside to this code is that it requires a round trip to the browser: the browser requested the show action with <code>/books/1</code> 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 <code>/books/</code>, the browser complies and sends a new request back to the controller asking now for the <code>index</code> 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.</p><p>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:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def index
@books = Book.all
end
def show
@book = Book.find_by(id: params[:id])
if @book.nil?
@books = Book.all
flash.now[:alert] = "Your book was not found"
render "index"
end
end
</pre>
</div>
<p>This would detect that there are no books with the specified ID, populate the <code>@books</code> instance variable with all the books in the model, and then directly render the <code>index.html.erb</code> template, returning it to the browser with a flash alert message to tell the user what happened.</p><h4 id="using-head-to-build-header-only-responses"><a class="anchorlink" href="#using-head-to-build-header-only-responses">2.4 Using <code>head</code> To Build Header-Only Responses</a></h4><p>The <code>head</code> method can be used to send responses with only headers to the browser. The <code>head</code> method accepts a number or symbol (see <a href="#the-status-option">reference table</a>) representing an HTTP status code. The options argument is interpreted as a hash of header names and values. For example, you can return only an error header:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
head :bad_request