Action View Partials

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 a template for Advertiser#account:

<%= render partial: "account" %>

This would render “advertiser/_account.html.erb”.

In another template for Advertiser#buy, we could have:

<%= render partial: "account", locals: { account: @buyer } %>

<% @advertisements.each do |ad| %>
  <%= render partial: "ad", locals: { ad: ad } %>
<% end %>

This would first render “advertiser/_account.html.erb” with @buyer passed in as the local variable account, then render “advertiser/_ad.html.erb” and pass the local variable ad to the template for display.

The :as and :object options

By default ActionView::PartialRenderer doesn't have any local variables. The :object option can be used to pass an object to the partial. For instance:

<%= render partial: "account", object: @buyer %>

would provide the @buyer object to the partial, available under the local variable account and is equivalent to:

<%= render partial: "account", locals: { account: @buyer } %>

With the :as option we can specify a different name for said local variable. For example, if we wanted it to be user instead of account we'd do:

<%= render partial: "account", object: @buyer, as: 'user' %>

This is equivalent to

<%= render partial: "account", locals: { user: @buyer } %>

Rendering a collection of partials

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 in “Using partials” can be rewritten with a single line:

<%= render partial: "ad", collection: @advertisements %>

This will render “advertiser/_ad.html.erb” and pass the local variable ad to the template for display. An iteration object will automatically be made available to the template with a name of the form partial_name_iteration. The iteration object has knowledge about which index the current object has in the collection and the total size of the collection. The iteration object also has two convenience methods, first? and last?. In the case of the example above, the template would be fed ad_iteration. For backwards compatibility the partial_name_counter is still present and is mapped to the iteration's index method.

The :as option may be used when rendering partials.

You can specify a partial to be rendered between elements via the :spacer_template option. The following example will render advertiser/_ad_divider.html.erb between each ad partial:

<%= render partial: "ad", collection: @advertisements, spacer_template: "ad_divider" %>

If the given :collection is nil or empty, render will return nil. This will allow you to specify a text which will displayed instead by using this form:

<%= render(partial: "ad", collection: @advertisements) || "There's no ad to be displayed" %>

NOTE: Due to backwards compatibility concerns, the collection can't be one of hashes. Normally you'd also just keep domain objects, like Active Records, in there.

Rendering shared partials

Two controllers can share a set of partials and render them like this:

<%= render partial: "advertisement/ad", locals: { ad: @advertisement } %>

This will render the partial “advertisement/_ad.html.erb” regardless of which controller this is being called from.

Rendering objects that respond to `to_partial_path`

Instead of explicitly naming the location of a partial, you can also let PartialRenderer do the work and pick the proper path by checking `to_partial_path` method.

# @account.to_partial_path returns 'accounts/account', so it can be used to replace:
# <%= render partial: "accounts/account", locals: { account: @account} %>
<%= render partial: @account %>

# @posts is an array of Post instances, so every post record returns 'posts/post' on `to_partial_path`,
# that's why we can replace:
# <%= render partial: "posts/post", collection: @posts %>
<%= render partial: @posts %>

Rendering the default case

If you're not going to be using any of the options like collections or layouts, you can also use the short-hand defaults of render to render partials. Examples:

# Instead of <%= render partial: "account" %>
<%= render "account" %>

# Instead of <%= render partial: "account", locals: { account: @buyer } %>
<%= render "account", account: @buyer %>

# @account.to_partial_path returns 'accounts/account', so it can be used to replace:
# <%= render partial: "accounts/account", locals: { account: @account} %>
<%= render @account %>

# @posts is an array of Post instances, so every post record returns 'posts/post' on `to_partial_path`,
# that's why we can replace:
# <%= render partial: "posts/post", collection: @posts %>
<%= render @posts %>

Rendering partials with layouts

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. Imagine a list with two types of users:

<%# app/views/users/index.html.erb %>
Here's the administrator:
<%= render partial: "user", layout: "administrator", locals: { user: administrator } %>

Here's the editor:
<%= render partial: "user", layout: "editor", locals: { user: editor } %>

<%# app/views/users/_user.html.erb %>
Name: <%= user.name %>

<%# app/views/users/_administrator.html.erb %>
<div id="administrator">
  Budget: $<%= user.budget %>
  <%= yield %>
</div>

<%# app/views/users/_editor.html.erb %>
<div id="editor">
  Deadline: <%= user.deadline %>
  <%= yield %>
</div>

…this will return:

Here's the administrator:
<div id="administrator">
  Budget: $<%= user.budget %>
  Name: <%= user.name %>
</div>

Here's the editor:
<div id="editor">
  Deadline: <%= user.deadline %>
  Name: <%= user.name %>
</div>

If a collection is given, the layout will be rendered once for each item in the collection. For example, these two snippets have the same output:

<%# app/views/users/_user.html.erb %>
Name: <%= user.name %>

<%# app/views/users/index.html.erb %>
<%# This does not use layouts %>
<ul>
  <% users.each do |user| -%>
    <li>
      <%= render partial: "user", locals: { user: user } %>
    </li>
  <% end -%>
</ul>

<%# app/views/users/_li_layout.html.erb %>
<li>
  <%= yield %>
</li>

<%# app/views/users/index.html.erb %>
<ul>
  <%= render partial: "user", layout: "li_layout", collection: users %>
</ul>

Given two users whose names are Alice and Bob, these snippets return:

<ul>
  <li>
    Name: Alice
  </li>
  <li>
    Name: Bob
  </li>
</ul>

The current object being rendered, as well as the object_counter, will be available as local variables inside the layout template under the same names as available in the partial.

You can also apply a layout to a block within any template:

<%# app/views/users/_chief.html.erb %>
<%= render(layout: "administrator", locals: { user: chief }) do %>
  Title: <%= chief.title %>
<% end %>

…this will return:

<div id="administrator">
  Budget: $<%= user.budget %>
  Title: <%= chief.name %>
</div>

As you can see, the :locals hash is shared between both the partial and its layout.

If you pass arguments to “yield” then this will be passed to the block. One way to use this is to pass an array to layout and treat it as an enumerable.

<%# app/views/users/_user.html.erb %>
<div class="user">
  Budget: $<%= user.budget %>
  <%= yield user %>
</div>

<%# app/views/users/index.html.erb %>
<%= render layout: @users do |user| %>
  Title: <%= user.title %>
<% end %>

This will render the layout for each user and yield to the block, passing the user, each time.

You can also yield multiple times in one layout and use block arguments to differentiate the sections.

<%# app/views/users/_user.html.erb %>
<div class="user">
  <%= yield user, :header %>
  Budget: $<%= user.budget %>
  <%= yield user, :footer %>
</div>

<%# app/views/users/index.html.erb %>
<%= render layout: @users do |user, section| %>
  <%- case section when :header -%>
    Title: <%= user.title %>
  <%- when :footer -%>
    Deadline: <%= user.deadline %>
  <%- end -%>
<% end %>