Action View templates are written using a mixture of HTML and eRuby tags. eRuby is short for embedded Ruby and that’s pretty much all the magic there is to it. Plain Ruby inserted into HTML (or XML or something else). You trigger eRuby by using embeddings such as <% %> and <%= %>. The difference is whether you want output or not. Consider the following loop for names:
<b>Names of all the people</b>
<% for person in @people %>
Name: <%= person.name %><br/>
<% end %>
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. Regular output functions like print or puts won’t work with eRuby templates. So this would be wrong:
Hi, Mr. <% puts "Frodo" %>
(If you absolutely must write from within a function, you can use the TextHelper#concat)
Using sub templates allows you to sidestep tedious replication and extract common display structures in shared templates. The classic example is the use of a header and footer (even though the Action Pack-way would be to use Layouts):
<%= render "shared/header" %> Something really specific and terrific <%= render "shared/footer" %>
As you see, we use the output embeddings for the render methods. The render call itself will just return a string holding the result of the rendering. The output embedding writes it to the current template.
But you don’t have to restrict yourself to static includes. Templates can share variables amongst themselves by using instance variables defined in using the regular embedding tags. Like this:
<% @page_title = "A Wonderful Hello" %> <%= render "shared/header" %>
Now the header can pick up on the @page_title variable and use it for outputting a title tag:
<title><%= @page_title %></title>
You can pass local variables to sub templates by using a hash of with the variable names as keys and the objects as values:
<%= render "shared/header", { "headline" => "Welcome", "person" => person } %>
These can now be accessed in shared/header with:
Headline: <%= headline %> First name: <%= person.first_name %>
There’s also a convenience method for rendering sub templates within the current controller that depends on a single object (we call this kind of sub templates for partials). It relies on the fact that partials should follow the naming convention of being prefixed with an underscore — as to separate them from regular templates that could be rendered on their own. In the template for Advertiser#buy, we could have:
<% for ad in @advertisements %>
<%= render_partial "ad", ad %>
<% end %>
This would render "advertiser/_ad.rhtml" and pass the local variable ad for the template to display.
The example of partial use describes a familar 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 of as the elements contained within. So the three-lined example in "Using partials" can be rewritten with a single line:
<%= render_collection_of_partials "ad", @advertisements %>
So this will render "advertiser/_ad.rhtml" and pass the local variable ad for the template to display.
The ERbTemplate is the default Action View class used by the Action Controller. If you want to use a different class, you’ll need to implement this interface, and use the Base.template_class=. There is already one other implementation available. That’s the ErubyTemplate, which is functionally identical to the default ERbTemplate, but uses the C-version of eRuby.