Passing Variables to View Partials

Sometimes the multiple ways that variables are passed to partials confuses me.

locals

Using locals, whenever you render a partial you must declare the same variable. If you attempt to render the same partial in a different view template without declaring the same local variable, you’ll get an error.

<%= render :partial => 'form', :locals => { :post => @post } %>

local_assigns

In cases where sometimes you want to pass a variable to a partial but other times you don’t, use local_assigns. Be aware that you will need to check for the existense of a variable in the partial before you use it.

<%= render article, full: true %>
<!-- in template -->
<h2><%= article.title %></h2>

<% if local_assigns[:full] %>
  <%= simple_format article.body %>
<% else %>
  <%= truncate article.body %>
<% end %>

object

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 :object option:

<%= render partial: "customer", object: @new_customer %>

If you see the super shorthand syntax:

<%= render @customer %>

it uses object: @customer.

Local Variables in Collections

These work essentially the same as with single object view partials. To customize the name of the local variable, use the :as option:

<%= render partial: "product", collection: @products, as: :item %>

You can also still create local variables using locals like before:

<%= render partial: "product", collection: @products,
           as: :item, locals: {title: "Products Page"} %>

Read more in the Rails Guide.