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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. Show 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. From the controller's point of view, there are three ways to create an HTTP response:
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 03 class: And the following in your routes file: And you have a view file 04: Rails will automatically render 04 when you navigate to 06 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 07 model and add the index action to 03: 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 09 template in the controller's view path and render it. So in this case, Rails will render the 04 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: The actual rendering is done by nested classes of the module 11. 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. In most cases, the controller's 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 00. You can render the default view for a Rails template, or a specific template, or a file, or inline code, or nothing at all. You can render text, JSON, or XML. You can specify the content type or HTTP status of the rendered response as well. If you want to see the exact results of a call to 00 without needing to inspect it in a browser, you can call 16. This method takes exactly the same options as 00, but it returns a string instead of sending a response back to the browser. If you want to render the view that corresponds to a different template within the same controller, you can use 00 with the name of the view: If the call to 19 fails, calling the 19 action in this controller will render the 21 template belonging to the same controller. If you prefer, you can use a symbol instead of a string to specify the action to render: 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 00, which accepts the full path (relative to
24 that lives in 25, you can render the results of an action to a template in 26 this way: 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 27 option (which was required on Rails 2.2 and earlier): 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 03 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 21 template in the 30 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. The 00 method can do without a view completely, if you're willing to use the 32 option to supply ERB as part of the method call. This is perfectly valid: 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 35 option: You can send plain text - with no markup at all - back to the browser by using the 36 option to 00: Rendering pure text is most useful when you're responding to Ajax or web service requests that are expecting something other than proper HTML. By default, if you use the 36 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 39 option and use the 40 extension for the layout file. You can send an HTML string back to the browser by using the 41 option to 00: 0 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. When using 43 option, HTML entities will be escaped if the string is not composed with 44-aware APIs. 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: You don't need to call 45 on the object that you want to render. If you use the 46 option, 00 will automatically call 45 for you. Rails also has built-in support for converting objects to XML and rendering that XML back to the caller: You don't need to call 49 on the object that you want to render. If you use the 50 option, 00 will automatically call 49 for you. Rails can render vanilla JavaScript: 1 This will send the supplied string to the browser with a MIME type of 53. You can send a raw content back to the browser, without setting any content type, by using the 54 option to 00: This option should be used only if you don't care about the content type of the response. Using 36 or 41 might be more appropriate most of the time. Unless overridden, your response returned from this render option will be 58, as that is the default content type of Action Dispatch response. Rails can render a raw file from an absolute path. This is useful for conditionally rendering static files like error pages. 2 This renders the raw file (it doesn't support ERB or other handlers). By default it is rendered within the current layout. Using the 59 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. 60 is often a faster and better option if a layout isn't required. Rails can render objects responding to 61. This calls 62 on the provided object with the current view context. You can also provide the object by using the 63 option to 00: 3 Calls to the method generally accept six options:
By default, Rails will serve the results of a rendering operation with the MIME content-type of 74 (or 75 if you use the 46 option, or 77 for the 50 option.). There are times when you might like to change this, and you can do so by setting the 67 option: 4 With most of the options to 00, 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 68 option to tell Rails to use a specific file as the layout for the current action: 5 You can also tell Rails to render with no layout at all: You can use the 69 option to set the HTTP 85 header: 6 Rails will automatically generate a response with the correct HTTP status code (in most cases, this is 87). You can use the 70 option to change this: 7 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 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. Rails uses the format specified in the request (or 41 by default). You can change this passing the 71 option with a symbol or an array: 8 If a template with the specified format does not exist an 92 error is raised. This tells Rails to look for template variations of the same format. You can specify a list of variants by passing the 72 option with a symbol or an array. An example of use would be this. 9 With this set of variants Rails will look for the following set of templates and use the first that exists.
If a template with the specified format does not exist an 92 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. 0 To find the current layout, Rails first looks for a file in 99 with the same base name as the controller. For example, rendering actions from the 00 class will use 01 (or 02). If there is no such controller-specific layout, Rails will use 03 or 04. If there is no 05 layout, Rails will use a 06 layout if one exists. Rails also provides several ways to more precisely assign specific layouts to individual controllers and actions. You can override the default layout conventions in your controllers by using the declaration. For example: 1 With this declaration, all of the views rendered by the 08 will use 09 as their layout. To assign a specific layout for the entire application, use a 07 declaration in your 11 class: 2 With this declaration, all of the views in the entire application will use 12 for their layout. You can use a symbol to defer the choice of layout until a request is processed: 3 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 13 instance, so the layout can be determined based on the current request: 4 Layouts specified at the controller level support the 14 and 15 options. These options take either a method name, or an array of method names, corresponding to method names within the controller: 5 With this declaration, the 16 layout would be used for everything but the 17 and 18 methods. Layout declarations cascade downward in the hierarchy, and more specific layout declarations always override more general ones. For example:
In this application:
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: 0 1 2 The lookup order for an 31 action will be:
This makes 34 a great place for your shared partials, which can then be rendered in your ERB as such: 3 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 00 works. For example, here's some code that will trigger this error: 4 If 37 evaluates to 38, Rails will start the rendering process to dump the 39 variable into the 40 view. But this will not stop the rest of the code in the 41 action from running, and when Rails hits the end of the action, it will start to render the 42 view - and throw an error. The solution is simple: make sure that you have only one call to 00 or 44 in a single code path. One thing that can help is 45. Here's a patched version of the method: 5 Note that the implicit render done by ActionController detects if 00 has been called, so the following will work without errors: 6 This will render a book with 47 set with the 40 template, while other books will render with the default 41 template. Another way to handle returning responses to an HTTP request is with . As you've seen, 00 tells Rails which view (or other asset) to use in constructing a response. The 01 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: You can use to return the user to the page they just came from. This location is pulled from the 55 header which is not guaranteed to be set by the browser, so you must provide the 56 to use in this case. 7 01 and 54 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 45 or some other halting mechanism, if needed. Rails uses HTTP status code 302, a temporary redirect, when you call 01. If you'd like to use a different status code, perhaps 301, a permanent redirect, you can use the 70 option: 8 Just like the 70 option for 00, 70 for 01 accepts both numeric and symbolic header designations. Sometimes inexperienced developers think of 01 as a sort of 69 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: 9 With the code in this form, there will likely be a problem if the 39 variable is 71. Remember, a 72 doesn't run any code in the target action, so nothing will set up the 73 variable that the 18 view will probably require. One way to fix this is to redirect instead of rendering: 0 With this code, the browser will make a fresh request for the index page, the code in the 18 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 76 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 77, the browser complies and sends a new request back to the controller asking now for the 18 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: 1 This would detect that there are no books with the specified ID, populate the 73 instance variable with all the books in the model, and then directly render the 80 template, returning it to the browser with a flash alert message to tell the user what happened. The method can be used to send responses with only headers to the browser. The 02 method accepts a number or symbol (see ) 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: This would produce the following header: 2 Or you can use other HTTP headers to convey other information: 3 Which would produce: 4 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 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: You can use these tags in layouts or other views, although the 85, 86, and 87, are most commonly used in the 94 section of a layout. The asset tag helpers do not verify the existence of the assets at the specified locations; they simply assume that you know what you're doing and generate the link. The 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 ( 97, 98, or 46), a hash of options that are passed through to url_for, and a hash of options for the tag: 5 There are three tag options available for the 85:
The helper returns an HTML 08 tag for each source provided. If you are using Rails with the Asset Pipeline enabled, this helper will generate a link to 09 rather than 10 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: 11, 12 or 13. These locations are explained in detail in the . 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 14 inside of one of 11, 12 or 13, you would do this: 6 Rails will then output a 08 tag such as this: 7 The request to this asset is then served by the Sprockets gem. To include multiple files such as 19 and 20 at the same time: 8 To include 19 and 22: 9 To include 23: 0 The helper returns an HTML 26 tag for each source provided. If you are using Rails with the "Asset Pipeline" enabled, this helper will generate a link to 27. This link is then processed by the Sprockets gem. A stylesheet file can be stored in one of three locations: 11, 12, or 13. 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 31 inside of one of 11, 12, or 13, you would do this: 1 To include 35 and 36: 2 To include 35 and 38: 3 To include 39: 4 By default, the 87 creates links with 41. You can override this default by specifying an appropriate option ( 01): 5 The helper builds an HTML 45 tag to the specified file. By default, files are loaded from 46. Note that you must specify the extension of the image. 6 You can supply a path to the image if you like: 7 You can supply a hash of additional HTML options: 8 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: 9 You can also specify a special size tag, in the format "{width}x{height}": 0 In addition to the above special tags, you can supply a final hash of standard HTML options, such as 47, 48, or 49: 1 The helper builds an HTML5 52 tag to the specified file. By default, files are loaded from 53. 2 Produces 3 Like an 88 you can supply a path, either absolute, or relative to the 53 directory. Additionally you can specify the 56 option just like an 88. Video tags can also have any of the HTML options specified at the end ( 58, 59 et al). The video tag also supports all of the 52 HTML options through the HTML options hash, including:
You can also specify multiple videos to play by passing an array of videos to the 89: 4 This will produce: 5 The helper builds an HTML5 69 tag to the specified file. By default, files are loaded from 70. 6 You can supply a path to the audio file if you like: 7 You can also supply a hash of additional options, such as 48, 47, etc. Like the 89, the 90 has special options:
Within the context of a layout, 83 identifies a section where content from the view should be inserted. The simplest way to use this is to have a single 83, into which the entire contents of the view currently being rendered is inserted: 8 You can also create a layout with multiple yielding regions: 9 The main body of the view will always render into the unnamed 83. To render content into a named 83, you use the 84 method. The method allows you to insert content into a named 83 block in your layout. For example, this view would work with the layout that you just saw: 0 The result of rendering this page into the supplied layout would be this HTML: 1 The 84 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. 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. To render a partial as part of a view, you use the method within the view: This will render a file named 89 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: 2 Since view partials rely on the same as templates and layouts, that code will pull in the partial from 90. 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: 3 Here, the 91 and 92 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, 83 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:
For content that is shared among all pages in your application, you can use partials directly from 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: 7 This would look for a partial named 97 and render it using the layout 98. 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 99 folder). Also note that explicitly specifying 00 is required when passing additional options such as 68. 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. To pass a local variable to a partial in only specific cases use the 05.
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 09 option: 4 Within the 10 partial, the 10 variable will refer to 12 from the parent view. If you have an instance of a model to render into a partial, you can use a shorthand syntax: Assuming that the 13 instance variable contains an instance of the 14 model, this will use 15 to render it and will pass the local variable 10 into the partial which will refer to the 13 instance variable in the parent view. Partials are very useful in rendering collections. When you pass a collection to a partial via the 18 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 21, and within the 21 partial, you can refer to 16 to get the instance that is being rendered. There is also a shorthand for this. Assuming 24 is a collection of 25 instances, you can simply write this in the 80 to produce the same result: 7 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:
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, 00 will return nil, so it should be fairly simple to provide alternative content. 1 To use a custom local variable name within the partial, specify the 31 option in the call to the partial: 2 With this change, you can access an instance of the 24 collection as the 33 local variable within the partial. You can also pass in arbitrary local variables to any partial you are rendering with the 34 option: 3 In this case, the partial will have access to a local variable 35 with the value "Products Page". Rails also makes a counter variable available within a partial called by the collection. The variable is named after the title of the partial followed by 36. For example, when rendering a collection 24 the partial 20 can access the variable 39. The variable indexes the number of times the partial has been rendered within the enclosing view, starting with a value of 40 on the first render. 4 5 This also works when the partial name is changed using the 41 option. So if you did 42, the counter variable would be 43. You can also specify a second partial to be rendered between instances of the main partial by using the 44 option: 6 Rails will render the 45 partial (with no data passed in to it) between each pair of 21 partials. When rendering collections it is also possible to use the 68 option: 7 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. 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 11 layout:
On pages generated by 50, you want to hide the top menu and add a right menu:
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 52 method via 53 to base a new layout on the News layout. If you are sure you will not subtemplate the 54 layout, you can replace the 55 with simply 83. FeedbackYou're encouraged to help improve the quality of this guide. Please contribute if you see any typos or factual errors. To get started, you can read our section. You may also find incomplete content or stuff that is not up to date. Please do add any missing documentation for main. Make sure to check Edge Guides first to verify if the issues are already fixed or not on the main branch. Check the Ruby on Rails Guides Guidelines for style and conventions. If for whatever reason you spot something to fix but cannot patch it yourself, please open an issue. And last but not least, any kind of discussion regarding Ruby on Rails documentation is very welcome on the official Ruby on Rails Forum. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
