Added Partials section to ActionView guide.

1 files changed, 95 insertions, 2 deletions

diff --git a/railties/guides/source/action_view_overview.textile b/railties/guides/source/action_view_overview.textile
index 33735b7944..72fcb8c0e6 100644
--- a/railties/guides/source/action_view_overview.textile
+++ b/railties/guides/source/action_view_overview.textile
@@ -115,9 +115,102 @@ TODO needs a screenshot? I have one - not sure where to put it.
 
 h3. Templates, Partials and Layouts
 
-TODO...
+As mentioned before, the final HTML output is a composition of three Rails elements: +Templates+, +Partials+ and +Layouts+.
+Find below a brief overview of each one of them.
+
+h4. 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.
+
+h5. Naming Partials
+
+To render a partial as part of a view, you use the +render+ method within the view:
+
+<ruby>
+<%= render "menu" %>
+</ruby>
+
+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:
+
+<ruby>
+<%= render "shared/menu" %>
+</ruby>
+
+That code will pull in the partial from +app/views/shared/_menu.html.erb+.
+
+h5. 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>
+<% @products.each do |product| %>
+  <%= render :partial => "product", :locals => { :product => product } %>
+<% end %>
+
+<%= render "shared/footer" %>
+</erb>
+
+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.
+
+h5. The :as and :object options
+
+By default <tt>ActionView::Partials::PartialRenderer</tt> has its object in a local variable with the same name as the template. So, given
+
+<erb>
+<%= render :partial => "product" %>
+</erb>
+
+within product we'll get <tt>@product</tt> in the local variable +product+, as if we had written:
+
+<erb>
+<%= render :partial => "product", :locals => { :product  => @product } %>
+</erb>
+
+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:
+
+<erb>
+<%= render :partial => "product", :as => 'item' %>
+</erb>
+
+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.
+
+For example, instead of:
+
+<erb>
+<%= render :partial => "product", :locals => { :product  => @item } %>
+</erb>
+
+you'd do:
+
+<erb>
+<%= render :partial => "product", :object => @item %>
+</erb>
+
+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:
+
+<erb>
+<%= render :partial => "product", :collection => @products %>
+</erb>
+
+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:
+
+<erb>
+<%= render @products %>
+</erb>
 
-TODO see http://guides.rubyonrails.org/layouts_and_rendering.html
+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.
 
 h3. Using Templates, Partials and Layouts in "The Rails Way"