Passing param values to redirect_to as querystring in rails

This should be simple, but I can't seem to find an easy answer.

How can I pass param values from the current request into a redirect_to call? I have some form values I'd like to pass into the query string of a GET redirect

I'd like to do something like:

redirect_to @thing, :foo => params[:foo]

and get sent to:

http://things/4?[foo][key1]=val1&[foo][key2]=val2

Thanks!

Also - how could this be handled for a redirect_to :back?

redirect_to :back, :foo => params[:foo]