HOWTO: Create a new link by munging the current URL in Rails

Mar 20 ’09

I've been working on a restful app that allows very complicated searches. I've been composing scopes to build the searches, but that's another post. All searches use the index action of the resource's controller. I wanted to offer RSS feeds for every possible search, to allow users to really get only the information they are interested. But I couldn't figure out how to generate the custom RSS feed link easily. I could have gone through my list of scopes, and then re-created the URL with :format => :rss, but that's not very DRY. It turns out that there's a much easier way.

Put the following (or something like it), in the <head> section of your output:

<%= auto_discovery_link_tag(:rss, {:overwrite_params => {:format => :rss}}, {:title => 'RSS feed for this search'}) %>

The magic here is the :overwrite_params parameter that gets passed into url_for to generate your URL. It tells url_for to preserve the current URL, exactly as it is, except for the changes specificed: in this case, switch the format to .rss. This is super-helpful behavior because it preserves GET params, like the arbitrary search parameters my users have input to get to this page.

Since :overwrite_params is used by url_for, any methods based on url_for can handle it: link_to is a great example.

Hooray for easy URL munging!