Upgrading to Rails 3: Beware of the Object#tap pattern

September 2nd, 2010 at 2:23 pm • permalink • 0 comments

The advent of Object#returning first, and Object#tap consequently, quickly lead to a very common and elegant pattern in the Rails community to scope a variable to a block in your template code.

Here’s a simple .erb file.

<% "Simone".tap |string| %>
Hello, <%= string %>.
<% end %>

Once executed, in Rails 2.3 the template renders the following content

Hello, Simone.

However, due to some internal changes in the way how Rails 3 handles block helpers, the same template in Rails 3 renders the following content.

Hello, Simone.Simone

Here’s a live example, yielding the String “USD“.

The template displays a USD string, after the Ruby block

The same statement also causes the following deprecation warning:

DEPRECATION WARNING: <% %> style block helpers are deprecated. Please use <%= %>.

The problem here is the Rails 3 compatibility layer which is responsible to help you migrate your code from Rails 2.3 to the new Rails 3 syntax. Apparently, whenever you are using a block which returns a not nil value, the compatibility layer classifies this behavior as a legacy block helper style.

Rails 3 showing deprecation warnings for Block Helpers

In the meantime of a real solution, you can use the following workaround.

(continue →)