Update guides/source/layouts_and_rendering.md Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>
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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:
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>
<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 %>
NOTE: The actual rendering is done by nested classes of the module ActionView::Template::Handlers
. 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.
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 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:
def update
@book = Book.find(params[:id])
if @book.update(book_params)
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:
def update
@book = Book.find(params[:id])
if @book.update(book_params)
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:
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"
Wrapping it up
The above two ways of rendering (rendering the template of another action in the same controller, and rendering the template of another action in a different controller) are actually variants of the same operation.
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 action: "edit"
render "books/edit"
render template: "books/edit"
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:
render inline: "<% products.each do |p| %><p><%= p.name %></p><% 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:
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 :plain
option to render
:
render plain: "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 :plain
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 and use the .text.erb
extension for the layout file.
Rendering HTML
You can send an HTML string back to the browser by using the :html
option to
render
:
render html: helpers.tag.strong('Not Found')
TIP: 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.
NOTE: When using html:
option, HTML entities will be escaped if the string is not composed with html_safe
-aware APIs.
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
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:
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:
render js: "alert('Hello Rails');"
This will send the supplied string to the browser with a MIME type of text/javascript
.
Rendering raw body
You can send a raw content back to the browser, without setting any content
type, by using the :body
option to render
:
render body: "raw"
TIP: This option should be used only if you don't care about the content type of
the response. Using :plain
or :html
might be more appropriate most of the
time.
NOTE: Unless overridden, your response returned from this render option will be
text/plain
, as that is the default content type of Action Dispatch response.
Rendering raw file
Rails can render a raw file from an absolute path. This is useful for conditionally rendering static files like error pages.
render file: "#{Rails.root}/public/404.html", layout: false
This renders the raw file (it doesn't support ERB or other handlers). By default it is rendered within the current layout.
WARNING: Using the :file
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.
TIP: send_file
is often a faster and better option if a layout isn't required.
Rendering objects
Rails can render objects responding to :render_in
.
render MyComponent.new
This calls render_in
on the provided object with the current view context.
Options for render
Calls to the render
method generally accept six options:
:content_type
:layout
:location
:status
:formats
:variants
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 template: "feed", 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:
render layout: "special_layout"
You can also tell Rails to render with no layout at all:
render layout: false
The :location
Option
You can use the :location
option to set the HTTP Location
header:
render xml: photo, location: photo_url(photo)
The :status
Option
Rails will automatically generate a response with the correct HTTP status code (in most cases, this is 200 OK
). You can use the :status
option to change this:
render status: 500
render status: :forbidden
Rails understands both numeric status codes and the corresponding symbols shown below.
Response Class | HTTP Status Code | Symbol |
---|---|---|
Informational | 100 | :continue |
101 | :switching_protocols | |
102 | :processing | |
Success | 200 | :ok |
201 | :created | |
202 | :accepted | |
203 | :non_authoritative_information | |
204 | :no_content | |
205 | :reset_content | |
206 | :partial_content | |
207 | :multi_status | |
208 | :already_reported | |
226 | :im_used | |
Redirection | 300 | :multiple_choices |
301 | :moved_permanently | |
302 | :found | |
303 | :see_other | |
304 | :not_modified | |
305 | :use_proxy | |
307 | :temporary_redirect | |
308 | :permanent_redirect | |
Client Error | 400 | :bad_request |
401 | :unauthorized | |
402 | :payment_required | |
403 | :forbidden | |
404 | :not_found | |
405 | :method_not_allowed | |
406 | :not_acceptable | |
407 | :proxy_authentication_required | |
408 | :request_timeout | |
409 | :conflict | |
410 | :gone | |
411 | :length_required | |
412 | :precondition_failed | |
413 | :payload_too_large | |
414 | :uri_too_long | |
415 | :unsupported_media_type | |
416 | :range_not_satisfiable | |
417 | :expectation_failed | |
421 | :misdirected_request | |
422 | :unprocessable_entity | |
423 | :locked | |
424 | :failed_dependency | |
426 | :upgrade_required | |
428 | :precondition_required | |
429 | :too_many_requests | |
431 | :request_header_fields_too_large | |
451 | :unavailable_for_legal_reasons | |
Server Error | 500 | :internal_server_error |
501 | :not_implemented | |
502 | :bad_gateway | |
503 | :service_unavailable | |
504 | :gateway_timeout | |
505 | :http_version_not_supported | |
506 | :variant_also_negotiates | |
507 | :insufficient_storage | |
508 | :loop_detected | |
510 | :not_extended | |
511 | :network_authentication_required |
NOTE: 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.
The :formats
Option
Rails uses the format specified in the request (or :html
by default). You can
change this passing the :formats
option with a symbol or an array:
render formats: :xml
render formats: [:json, :xml]
If a template with the specified format does not exist an ActionView::MissingTemplate
error is raised.
The :variants
Option
This tells Rails to look for template variations of the same format.
You can specify a list of variants by passing the :variants
option with a symbol or an array.
An example of use would be this.
# called in HomeController#index
render variants: [:mobile, :desktop]
With this set of variants Rails will look for the following set of templates and use the first that exists.
app/views/home/index.html+mobile.erb
app/views/home/index.html+desktop.erb
app/views/home/index.html.erb
If a template with the specified format does not exist an ActionView::MissingTemplate
error is raised.
Instead of setting the variant on the render call you may also set it on the request object in your controller action.
def index
request.variant = determine_variant
end
private
def determine_variant
variant = nil
# some code to determine the variant(s) to use
variant = :mobile if session[:use_mobile]
variant
end
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:
class ProductsController < ApplicationController
layout "inventory"
#...
end
With this declaration, all of the views rendered by the ProductsController
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:
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:
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
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.
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
-
articles_controller.rb
class ArticlesController < ApplicationController end
-
special_articles_controller.rb
class SpecialArticlesController < ArticlesController layout "special" end
-
old_articles_controller.rb
class OldArticlesController < SpecialArticlesController layout false def show @article = Article.find(params[:id]) end def index @old_articles = Article.older render layout: "old" end # ... end
In this application:
- In general, views will be rendered in the
main
layout ArticlesController#index
will use themain
layoutSpecialArticlesController#index
will use thespecial
layoutOldArticlesController#show
will use no layout at allOldArticlesController#index
will use theold
layout
Template Inheritance
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:
# 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
The lookup order for an admin/products#index
action will be:
app/views/admin/products/
app/views/admin/
app/views/application/
This makes app/views/application/
a great place for your shared partials, which can then be rendered in your ERB as such:
<%# 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>.
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.
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_back
to return the user to the page they just came from.
This location is pulled from the HTTP_REFERER
header which is not guaranteed
to be set by the browser, so you must provide the fallback_location
to use in this case.
redirect_back(fallback_location: root_path)
NOTE: redirect_to
and redirect_back
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 return
or some other halting mechanism, if needed.
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.
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 from 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
flash.now[: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. The head
method accepts a number or symbol (see reference table) 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:
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
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
andcontent_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 <head>
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 feed readers can use to detect the presence of RSS, Atom, or JSON feeds. It takes the type of the link (:rss
, :atom
, or :json
), 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 therel
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 uppercase: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 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.
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='/assets/main.js'></script>
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 http://example.com/main.js
:
<%= 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 <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
.
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:
<%= stylesheet_link_tag "main" %>
To include app/assets/stylesheets/main.css
and app/assets/stylesheets/columns.css
:
<%= stylesheet_link_tag "main", "columns" %>
To include app/assets/stylesheets/main.css
and app/assets/stylesheets/photos/columns.css
:
<%= stylesheet_link_tag "main", "photos/columns" %>
To include http://example.com/main.css
:
<%= stylesheet_link_tag "http://example.com/main.css" %>
By default, 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
):
<%= stylesheet_link_tag "main_print", media: "print" %>
Linking to Images with the image_tag
The image_tag
helper builds an HTML <img />
tag to the specified file. By default, files are loaded from public/images
.
WARNING: Note that you must specify the extension of the image.
<%= image_tag "header.png" %>
You can supply a path to the image if you like:
<%= image_tag "icons/delete.gif" %>
You can supply a hash of additional HTML options:
<%= image_tag "icons/delete.gif", {height: 45} %>
You can supply alternate text for the image which will be used if the user has images turned off in their browser. If you do not specify an alt text explicitly, it defaults to the file name of the file, capitalized and with no extension. For example, these two image tags would return the same code:
<%= image_tag "home.gif" %>
<%= image_tag "home.gif", alt: "Home" %>
You can also specify a special size tag, in the format "{width}x{height}":
<%= image_tag "home.gif", size: "50x20" %>
In addition to the above special tags, you can supply a final hash of standard HTML options, such as :class
, :id
or :name
:
<%= image_tag "home.gif", alt: "Go Home",
id: "HomeImage",
class: "nav_bar" %>
Linking to Videos with the video_tag
The video_tag
helper builds an HTML 5 <video>
tag to the specified file. By default, files are loaded from public/videos
.
<%= video_tag "movie.ogg" %>
Produces
<video src="/videos/movie.ogg" />
Like an image_tag
you can supply a path, either absolute, or relative to the public/videos
directory. Additionally you can specify the size: "#{width}x#{height}"
option just like an image_tag
. Video tags can also have any of the HTML options specified at the end (id
, class
et al).
The video tag also supports all of the <video>
HTML options through the HTML options hash, including:
poster: "image_name.png"
, provides an image to put in place of the video before it starts playing.autoplay: true
, starts playing the video on page load.loop: true
, loops the video once it gets to the end.controls: true
, provides browser supplied controls for the user to interact with the video.autobuffer: true
, the video will pre load the file for the user on page load.
You can also specify multiple videos to play by passing an array of videos to the video_tag
:
<%= video_tag ["trailer.ogg", "movie.ogg"] %>
This will produce:
<video>
<source src="/videos/trailer.ogg">
<source src="/videos/movie.ogg">
</video>
Linking to Audio Files with the audio_tag
The audio_tag
helper builds an HTML 5 <audio>
tag to the specified file. By default, files are loaded from public/audios
.
<%= audio_tag "music.mp3" %>
You can supply a path to the audio file if you like:
<%= audio_tag "music/first_song.mp3" %>
You can also supply a hash of additional options, such as :id
, :class
etc.
Like the video_tag
, the audio_tag
has special options:
autoplay: true
, starts playing the audio on page loadcontrols: true
, provides browser supplied controls for the user to interact with the audio.autobuffer: true
, the audio will pre load the file for the user on page load.
Understanding yield
Within the context of a layout, yield
identifies a section where content from the view should be inserted. The simplest way to use this is to have a single yield
, into which the entire contents of the view currently being rendered is inserted:
<html>
<head>
</head>
<body>
<%= yield %>
</body>
</html>
You can also create a layout with multiple yielding regions:
<html>
<head>
<%= yield :head %>
</head>
<body>
<%= yield %>
</body>
</html>
The main body of the view will always render into the unnamed yield
. To render content into a named yield
, you use the content_for
method.
Using the content_for
Method
The content_for
method allows you to insert content into a named yield
block in your layout. For example, this view would work with the layout that you just saw:
<% content_for :head do %>
<title>A simple page</title>
<% end %>
<p>Hello, Rails!</p>
The result of rendering this page into the supplied layout would be this HTML:
<html>
<head>
<title>A simple page</title>
</head>
<body>
<p>Hello, Rails!</p>
</body>
</html>
The content_for
method is very helpful when your layout contains distinct regions such as sidebars and footers that should get their own blocks of content inserted. It's also useful for inserting tags that load page-specific JavaScript or css files into the header of an otherwise generic layout.
Using Partials
Partial templates - usually just called "partials" - are another device for breaking the rendering process into more manageable chunks. With a partial, you can move the code for rendering a particular piece of a response to its own file.
Naming Partials
To render a partial as part of a view, you use the render
method within the view:
<%= render "menu" %>
This will render a file named _menu.html.erb
at that point within the view being rendered. Note the leading underscore character: partials are named with a leading underscore to distinguish them from regular views, even though they are referred to without the underscore. This holds true even when you're pulling in a partial from another folder:
<%= render "shared/menu" %>
That code will pull in the partial from app/views/shared/_menu.html.erb
.
Using Partials to Simplify Views
One way to use partials is to treat them as the equivalent of subroutines: as a way to move details out of a view so that you can grasp what's going on more easily. For example, you might have a view that looked like this:
<%= render "shared/ad_banner" %>
<h1>Products</h1>
<p>Here are a few of our fine products:</p>
...
<%= render "shared/footer" %>
Here, the _ad_banner.html.erb
and _footer.html.erb
partials could contain
content that is shared by many pages in your application. You don't need to see
the details of these sections when you're concentrating on a particular page.
As seen in the previous sections of this guide, yield
is a very powerful tool
for cleaning up your layouts. Keep in mind that it's pure Ruby, so you can use
it almost everywhere. For example, we can use it to DRY up form layout
definitions for several similar resources:
-
users/index.html.erb
<%= render "shared/search_filters", search: @q do |form| %> <p> Name contains: <%= form.text_field :name_contains %> </p> <% end %>
-
roles/index.html.erb
<%= render "shared/search_filters", search: @q do |form| %> <p> Title contains: <%= form.text_field :title_contains %> </p> <% end %>
-
shared/_search_filters.html.erb
<%= form_with model: search do |form| %> <h1>Search form:</h1> <fieldset> <%= yield form %> </fieldset> <p> <%= form.submit "Search" %> </p> <% end %>
TIP: For content that is shared among all pages in your application, you can use partials directly from layouts.
Partial Layouts
A partial can use its own layout file, just as a view can use a layout. For example, you might call a partial like this:
<%= render partial: "link_area", layout: "graybar" %>
This would look for a partial named _link_area.html.erb
and render it using the layout _graybar.html.erb
. Note that layouts for partials follow the same leading-underscore naming as regular partials, and are placed in the same folder with the partial that they belong to (not in the master layouts
folder).
Also note that explicitly specifying :partial
is required when passing additional options such as :layout
.
Passing Local Variables
You can also pass local variables into partials, making them even more powerful and flexible. For example, you can use this technique to reduce duplication between new and edit pages, while still keeping a bit of distinct content:
-
new.html.erb
<h1>New zone</h1> <%= render partial: "form", locals: {zone: @zone} %>
-
edit.html.erb
<h1>Editing zone</h1> <%= render partial: "form", locals: {zone: @zone} %>
-
_form.html.erb
<%= form_with model: zone do |form| %> <p> <b>Zone name</b><br> <%= form.text_field :name %> </p> <p> <%= form.submit %> </p> <% end %>
Although the same partial will be rendered into both views, Action View's submit helper will return "Create Zone" for the new action and "Update Zone" for the edit action.
To pass a local variable to a partial in only specific cases use the local_assigns
.
-
index.html.erb
<%= render user.articles %>
-
show.html.erb
<%= render article, full: true %>
-
_article.html.erb
<h2><%= article.title %></h2> <% if local_assigns[:full] %> <%= simple_format article.body %> <% else %> <%= truncate article.body %> <% end %>
This way it is possible to use the partial without the need to declare all local variables.
Every partial also has a local variable with the same name as the partial (minus the leading underscore). You can pass an object in to this local variable via the :object
option:
<%= render partial: "customer", object: @new_customer %>
Within the customer
partial, the customer
variable will refer to @new_customer
from the parent view.
If you have an instance of a model to render into a partial, you can use a shorthand syntax:
<%= render @customer %>
Assuming that the @customer
instance variable contains an instance of the Customer
model, this will use _customer.html.erb
to render it and will pass the local variable customer
into the partial which will refer to the @customer
instance variable in the parent view.
Rendering Collections
Partials are very useful in rendering collections. When you pass a collection to a partial via the :collection
option, the partial will be inserted once for each member in the collection:
-
index.html.erb
<h1>Products</h1> <%= render partial: "product", collection: @products %>
-
_product.html.erb
<p>Product Name: <%= product.name %></p>
When a partial is called with a pluralized collection, then the individual instances of the partial have access to the member of the collection being rendered via a variable named after the partial. In this case, the partial is _product
, and within the _product
partial, you can refer to product
to get the instance that is being rendered.
There is also a shorthand for this. Assuming @products
is a collection of Product
instances, you can simply write this in the index.html.erb
to produce the same result:
<h1>Products</h1>
<%= render @products %>
Rails determines the name of the partial to use by looking at the model name in the collection. In fact, you can even create a heterogeneous collection and render it this way, and Rails will choose the proper partial for each member of the collection:
-
index.html.erb
<h1>Contacts</h1> <%= render [customer1, employee1, customer2, employee2] %>
-
customers/_customer.html.erb
<p>Customer: <%= customer.name %></p>
-
employees/_employee.html.erb
<p>Employee: <%= employee.name %></p>
In this case, Rails will use the customer or employee partials as appropriate for each member of the collection.
In the event that the collection is empty, render
will return nil, so it should be fairly simple to provide alternative content.
<h1>Products</h1>
<%= render(@products) || "There are no products available." %>
Local Variables
To use a custom local variable name within the partial, specify the :as
option in the call to the partial:
<%= render partial: "product", collection: @products, as: :item %>
With this change, you can access an instance of the @products
collection as the item
local variable within the partial.
You can also pass in arbitrary local variables to any partial you are rendering with the locals: {}
option:
<%= render partial: "product", collection: @products,
as: :item, locals: {title: "Products Page"} %>
In this case, the partial will have access to a local variable title
with the value "Products Page".
TIP: Rails also makes a counter variable available within a partial called by the collection, named after the title of the partial followed by _counter
. For example, when rendering a collection @products
the partial _product.html.erb
can access the variable product_counter
which indexes the number of times it has been rendered within the enclosing view. Note that it also applies for when the partial name was changed by using the as:
option. For example, the counter variable for the code above would be item_counter
.
You can also specify a second partial to be rendered between instances of the main partial by using the :spacer_template
option:
Spacer Templates
<%= render partial: @products, spacer_template: "product_ruler" %>
Rails will render the _product_ruler
partial (with no data passed in to it) between each pair of _product
partials.
Collection Partial Layouts
When rendering collections it is also possible to use the :layout
option:
<%= render partial: "product", collection: @products, layout: "special_layout" %>
The layout will be rendered together with the partial for each item in the collection. The current object and object_counter variables will be available in the layout as well, the same way they are within the partial.
Using Nested Layouts
You may find that your application requires a layout that differs slightly from your regular application layout to support one particular controller. Rather than repeating the main layout and editing it, you can accomplish this by using nested layouts (sometimes called sub-templates). Here's an example:
Suppose you have the following ApplicationController
layout:
-
app/views/layouts/application.html.erb
<html> <head> <title><%= @page_title or "Page Title" %></title> <%= stylesheet_link_tag "layout" %> <style><%= yield :stylesheets %></style> </head> <body> <div id="top_menu">Top menu items here</div> <div id="menu">Menu items here</div> <div id="content"><%= content_for?(:content) ? yield(:content) : yield %></div> </body> </html>
On pages generated by NewsController
, you want to hide the top menu and add a right menu:
-
app/views/layouts/news.html.erb
<% content_for :stylesheets do %> #top_menu {display: none} #right_menu {float: right; background-color: yellow; color: black} <% end %> <% content_for :content do %> <div id="right_menu">Right menu items here</div> <%= content_for?(:news_content) ? yield(:news_content) : yield %> <% end %> <%= render template: "layouts/application" %>
That's it. The News views will use the new layout, hiding the top menu and adding a new right menu inside the "content" div.
There are several ways of getting similar results with different sub-templating schemes using this technique. Note that there is no limit in nesting levels. One can use the ActionView::render
method via render template: 'layouts/news'
to base a new layout on the News layout. If you are sure you will not subtemplate the News
layout, you can replace the content_for?(:news_content) ? yield(:news_content) : yield
with simply yield
.