Action View and Action Controller are the two major components of Action Pack. In Rails, web requests are handled by Action Pack, which splits the work into a controller part (performing the logic) and a view part (rendering a template). Typically, Action Controller will be concerned with communicating with the database and performing CRUD actions where necessary. Action View is then responsible for compiling the response.
Action View templates are written using embedded Ruby in tags mingled with HTML. To avoid cluttering the templates with boilerplate code, a number of helper classes provide common behavior for forms, dates, and strings. It's also easy to add new helpers to your application as it evolves.
NOTE. Some features of Action View are tied to Active Record, but that doesn't mean that Action View depends on Active Record. Action View is an independent package that can be used with any sort of backend.
For each controller there is an associated directory in the <tt>app/views</tt> directory which holds the template files that make up the views associated with that controller. These files are used to display the view that results from each controller action.
Let's take a look at what Rails does by default when creating a new resource using the scaffold generator:
<shell>
$ rails generate scaffold post
[...]
invoke scaffold_controller
create app/controllers/posts_controller.rb
invoke erb
create app/views/posts
create app/views/posts/index.html.erb
create app/views/posts/edit.html.erb
create app/views/posts/show.html.erb
create app/views/posts/new.html.erb
create app/views/posts/_form.html.erb
[...]
</shell>
There is a naming convention for views in Rails. Typically, the views share their name with the associated controller action, as you can see above.
For example, the index controller action of the <tt>posts_controller.rb</tt> will use the <tt>index.html.erb</tt> view file in the <tt>app/views/posts</tt> directory.
The complete HTML returned to the client is composed of a combination of this ERB file, a layout template that wraps it, and all the partials that the view may reference. Later on this guide you can find a more detailed documentation of each one of this three components.
Action View works well with Action Record, but it can also be used with other Ruby tools. We can demonstrate this by creating a small "Rack":http://rack.rubyforge.org/ application that includes Action View functionality. This may be useful, for example, if you'd like access to Action View's helpers in a Rack application.
Action View templates can be written in several ways. If the template file has a <tt>.erb</tt> extension then it uses a mixture of ERB (included in Ruby) and HTML. If the template file has a <tt>.builder</tt> extension then a fresh instance of <tt>Builder::XmlMarkup</tt> library is used.
Rails supports multiple template systems and uses a file extension to distinguish amongst them. For example, an HTML file using the ERB template system will have <tt>.html.erb</tt> as a file extension.
h5. ERB
Within an ERB template Ruby code can be included using both +<% %>+ and +<%= %>+ tags. The +<% %>+ are used to execute Ruby code that does not return anything, such as conditions, loops or blocks, and the +<%= %>+ tags are used when you want output.
Consider the following loop for names:
<erb>
<b>Names of all the people</b>
<% @people.each do |person| %>
Name: <%= person.name %><br/>
<% end %>
</erb>
The loop is setup in regular embedding tags +<% %>+ and the name is written using the output embedding tag +<%= %>+. Note that this is not just a usage suggestion, for Regular output functions like print or puts won't work with ERB templates. So this would be wrong:
<erb>
<%# WRONG %>
Hi, Mr. <% puts "Frodo" %>
</erb>
To suppress leading and trailing whitespaces, you can use +<%-+ +-%>+ interchangeably with +<%+ and +%>+.
h5. Builder
Builder templates are a more programmatic alternative to ERB. They are especially useful for generating XML content. An XmlMarkup object named +xml+ is automatically made available to templates with a <tt>.builder</tt> extension.
By default, Rails will compile each template to a method in order to render it. When you alter a template, Rails will check the file's modification time and recompile it in development mode.
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.
h5. Naming Partials
To render a partial as part of a view, you use the +render+ method within the view:
This will render a file named +_menu.html.erb+ at that point within the view is 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:
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:
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.
With the <tt>:as</tt> option we can specify a different name for said local variable. For example, if we wanted it to be +item+ instead of product+ we'd do:
The <tt>:object</tt> option can be used to directly specify which object is rendered into the partial; useful when the template's object is elsewhere, in a different ivar or in a local variable for instance.
The <tt>:object</tt> and <tt>:as</tt> options can be used together.
h5. Rendering Collections
The example of partial use describes a familiar pattern where a template needs to iterate over an array and render a sub template for each of the elements. This pattern has been implemented as a single method that accepts an array and renders a partial by the same name as the elements contained within.
So the three-lined example for rendering all the products can be rewritten with a single line:
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.
You can use a shorthand syntax for rendering collections. Assuming @products is a collection of +Product+ instances, you can simply write the following 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.
Partials can have their own layouts applied to them. These layouts are different than the ones that are specified globally for the entire action, but they work in a similar fashion.
Let's say we're displaying a post on a page where it should be wrapped in a +div+ for display purposes. First, we'll create a new +Post+:
<ruby>
Post.create(:body => 'Partial Layouts are cool!')
</ruby>
In the +show+ template, we'll render the +post+ partial wrapped in the +box+ layout:
Note that the partial layout has access to the local +post+ variable that was passed into the +render+ call. However, unlike application-wide layouts, partial layouts still have the underscore prefix.
You can also render a block of code within a partial layout instead of calling +yield+. For example, if we didn't have the +post+ partial, we could do this instead:
The following is only a brief overview summary of the helpers available in Action View. It's recommended that you review the API Documentation, which covers all of the helpers in more detail, but this should serve as a good starting point.
The Active Record Helper makes it easier to create forms for records kept in instance variables. You may also want to review the "Rails Form helpers guide":form_helpers.html.
Returns a form with inputs for all attributes of the specified Active Record object. For example, let's say we have a +@post+ with attributes named +title+ of type +String+ and +body+ of type +Text+. Calling +form+ would produce a form to creating a new post with inputs for those attributes.
This module provides methods for generating a container tag, such as a +<div>+, for your record. This is the recommended way of creating a container for render your Active Record object, as it adds an appropriate class and id attributes to that container. You can then refer to those containers easily by following the convention, instead of having to think about which class or id attribute you should use.
h5. content_tag_for
Renders a container tag that relates to your Active Record Object.
For example, given +@post+ is the object of +Post+ class, you can do:
<ruby>
<%= content_tag_for(:tr, @post) do %>
<td><%= @post.title %></td>
<% end %>
</ruby>
This will generate this HTML output:
<html>
<tr id="post_1234" class="post">
<td>Hello World!</td>
</tr>
</html>
You can also supply HTML attributes as an additional option hash. For example:
<ruby>
<%= content_tag_for(:tr, @post, :class => "frontpage") do %>
You can pass a collection of Active Record objects. This method will loop through your objects and create a container for each of them. For example, given +@posts+ is an array of two +Post+ objects:
This is actually a convenient method which calls +content_tag_for+ internally with +:div+ as the tag name. You can pass either an Active Record object or a collection of objects. For example:
By default, Rails links to these assets on the current host in the public folder, but you can direct Rails to link to assets from a dedicated assets server by setting +ActionController::Base.asset_host+ in the application configuration, typically in +config/environments/production.rb+. For example, let's say your asset host is +assets.example.com+:
Register one or more JavaScript files to be included when symbol is passed to javascript_include_tag. This method is typically intended to be called from plugin initialization to register JavaScript files that the plugin installed in +public/javascripts+.
Register one or more stylesheet files to be included when symbol is passed to +stylesheet_link_tag+. This method is typically intended to be called from plugin initialization to register stylesheet files that the plugin installed in +public/stylesheets+.
Computes the path to an image asset in the +public/images+ directory. Full paths from the document root will be passed through. Used internally by +image_tag+ to build the image path.
Returns an html script tag for each of the sources provided. You can pass in the filename (+.js+ extension is optional) of JavaScript files that exist in your +public/javascripts+ directory for inclusion into the current page or you can pass the full path relative to your document root.
If the application does not use the asset pipeline, to include the jQuery JavaScript library in your application, pass +:defaults+ as the source. When using +:defaults+, if an +application.js+ file exists in your +public/javascripts+ directory, it will be included as well.
You can also cache multiple JavaScript files into one file, which requires less HTTP connections to download and can better be compressed by gzip (leading to faster transfers). Caching will only happen if +ActionController::Base.perform_caching+ is set to true (which is the case by default for the Rails production environment, but not for the development environment).
Computes the path to a JavaScript asset in the +public/javascripts+ directory. If the source filename has no extension, +.js+ will be appended. Full paths from the document root will be passed through. Used internally by +javascript_include_tag+ to build the script path.
You can also cache multiple stylesheets into one file, which requires less HTTP connections and can better be compressed by gzip (leading to faster transfers). Caching will only happen if ActionController::Base.perform_caching is set to true (which is the case by default for the Rails production environment, but not for the development environment).
Computes the path to a stylesheet asset in the +public/stylesheets+ directory. If the source filename has no extension, .css will be appended. Full paths from the document root will be passed through. Used internally by stylesheet_link_tag to build the stylesheet path.
Allows you to measure the execution time of a block in a template and records the result to the log. Wrap this block around expensive operations or possible bottlenecks to get a time reading for the operation.
A method for caching fragments of a view rather than an entire action or page. This technique is useful caching pieces like menus, lists of news topics, static HTML fragments, and so on. This method takes a block that contains the content you wish to cache. See +ActionController::Caching::Fragments+ for more information.
Calling +content_for+ stores a block of markup in an identifier for later use. You can make subsequent calls to the stored content in other templates or the layout by passing the identifier as an argument to +yield+.
For example, let's say we have a standard application layout, but also a special page that requires certain Javascript that the rest of the site doesn't need. We can use +content_for+ to include this Javascript on our special page without fattening up the rest of the site.
*app/views/layouts/application.html.erb*
<ruby>
<html>
<head>
<title>Welcome!</title>
<%= yield :special_script %>
</head>
<body>
<p>Welcome! The date and time is <%= Time.now %></p>
Returns a set of select tags (one for year, month, and day) pre-selected for accessing a specified date-based attribute.
<ruby>
date_select("post", "published_on")
</ruby>
h5. datetime_select
Returns a set of select tags (one for year, month, day, hour, and minute) pre-selected for accessing a specified datetime-based attribute.
<ruby>
datetime_select("post", "published_on")
</ruby>
h5. distance_of_time_in_words
Reports the approximate distance in time between two Time or Date objects or integers as seconds. Set +include_seconds+ to true if you want more detailed approximations.
<ruby>
distance_of_time_in_words(Time.now, Time.now + 15.seconds) # => less than a minute
distance_of_time_in_words(Time.now, Time.now + 15.seconds, true) # => less than 20 seconds
</ruby>
h5. select_date
Returns a set of html select-tags (one for year, month, and day) pre-selected with the +date+ provided.
<ruby>
# Generates a date select that defaults to the date provided (six days after today)
select_date(Time.today + 6.days)
# Generates a date select that defaults to today (no specified date)
Returns a select tag with options for each of the five years on each side of the current, which is selected. The five year radius can be changed using the +:start_year+ and +:end_year+ keys in the +options+.
Returns a set of select tags (one for hour, minute and optionally second) pre-selected for accessing a specified time-based attribute. The selects are prepared for multi-parameter assignment to an Active Record object.
<ruby>
# Creates a time select tag that, when POSTed, will be stored in the order variable in the submitted attribute
Form helpers are designed to make working with models much easier compared to using just standard HTML elements by providing a set of methods for creating forms based on your models. This helper generates the HTML for forms, providing a method for each sort of input (e.g., text, password, select, and so on). When the form is submitted (i.e., when the user hits the submit button or form.submit is called via JavaScript), the form inputs will be bundled into the params object and passed back to the controller.
There are two types of form helpers: those that specifically work with model attributes and those that don't. This helper deals with those that work with model attributes; to see an example of form helpers that don't work with model attributes, check the ActionView::Helpers::FormTagHelper documentation.
The core method of this helper, form_for, gives you the ability to create a form for a model instance; for example, let's say that you have a model Person and want to create a new instance of it:
<ruby>
# Note: a @person variable will have been created in the controller (e.g. @person = Person.new)
Creates a scope around a specific model object like form_for, but doesn't create the form tags themselves. This makes fields_for suitable for specifying additional model objects in the same form:
Returns a string of option tags for pretty much any country in the world.
h5. country_select
Return select and option tags for the given object and method, using country_options_for_select to generate the list of option tags.
h5. option_groups_from_collection_for_select
Returns a string of +option+ tags, like +options_from_collection_for_select+, but groups them by +optgroup+ tags based on the object relationships of the arguments.
Example object structure for use with this method:
Returns a string of option tags that have been compiled by iterating over the +collection+ and assigning the result of a call to the +value_method+ as the option value and the +text_method+ as the option text.
Provides a number of methods for creating form tags that doesn't rely on an Active Record object assigned to the template like FormHelper does. Instead, you provide the names and values manually.
Prior to Rails 3.1, if you are using file uploads, then you will need to set the multipart option for the form tag. Rails 3.1+ does this automatically.
Provides methods for converting numbers into formatted strings. Methods are provided for phone numbers, currency, percentage, precision, positional notation, and file size.
h5. number_to_currency
Formats a number into a currency string (e.g., $13.65).
For example, suppose you have a Posts controller with a show action. By default, calling this action will render +app/views/posts/show.html.erb+. But if you set +I18n.locale = :de+, then +app/views/posts/show.de.html.erb+ will be rendered instead. If the localized template isn't present, the undecorated version will be used. This means you're not required to provide localized views for all cases, but they will be preferred and used if available.
You can use the same technique to localize the rescue files in your public directory. For example, setting +I18n.locale = :de+ and creating +public/500.de.html+ and +public/404.de.html+ would allow you to have localized rescue pages.
Since Rails doesn't restrict the symbols that you use to set I18n.locale, you can leverage this system to display different content depending on anything you like. For example, suppose you have some "expert" users that should see different pages from "normal" users. You could add the following to +app/controllers/application.rb+:
