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
<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`:
```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:
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).
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:
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.
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.
#### 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):
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):
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:
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:
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:
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.
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:
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:
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.
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:
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:
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:
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:
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:
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:
* 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:
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:
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:
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.
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:
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:
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.
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 accepts a number or symbol (see [reference table](#the-status-option)) representing a 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:
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 `<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 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:
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
<scriptsrc='/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:
```erb
<%= javascript_include_tag "main", "columns" %>
```
To include `app/assets/javascripts/main.js` and `app/assets/javascripts/photos/columns.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:
```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`:
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`):
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:
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).
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+erb
<html>
<head>
</head>
<body>
<%= yield %>
</body>
</html>
```
You can also create a layout with multiple yielding regions:
```html+erb
<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:
```html+erb
<% 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+erb
<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:
```ruby
<%= 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:
```ruby
<%= 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:
```erb
<%= 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 among many pages in your application. You don't need to see the details of these sections when you're concentrating on a particular page.
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:
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:
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.
Every partial also has a local variable with the same name as the partial (minus the underscore). You can pass an object in to this local variable via the `:object` option:
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:
```erb
<%= 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:
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:
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:
Would render a partial `_products.html.erb` once for each instance of `product` in the `@products` instance variable passing the instance to the partial as a local variable called `item` and to each partial, make the local variable `title` available with the value `Products Page`.
TIP: Rails also makes a counter variable available within a partial called by the collection, named after the member of the collection followed by `_counter`. For example, if you're rendering `@products`, within the partial you can refer to `product_counter` to tell you how many times the partial has been rendered. This does not work in conjunction with the `as: :value` option.
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 do 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:
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`.
