[Rails] Rails 3 - Referencing the URL to determine if a Nav Item is class=“selected”

The app can have the following URLs:

<li> /projects/ </li>
<li> /projects/3 </li>
<li> /projects/3/photos </li>
<li> /projects/3/photos/312 </li>

I'd like to know how in Rails 3 I can look at the current URL and know
which of the lines above is currently active, so I can add a
class="selected"

Additional exmaple..

If the user's browser is on: /projects

And the Nav looks like

Projects
- Photos
- Files

Projects is Active

but if the user's browser is on: /files

Projects is active

and Files is active (they both apply).

Ideas? thanks

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