i thought maybe there was a change in rails 2.2 that would have made this work, but i just updated to 2.2.2 and this is still not working for me. here is some very simple code to demonstrate:

I have a ReportsController, with an view in the views/reports/ folder called "show.xml.builder"

In my controller:

x = render_to_string(:action => 'reports/show.xml.builder', :locals => {:blah => 'hi'})

In the view:

puts blah

I get a "undefined local variable or method blah'" error (because the local variable is never defined for the view).

:locals only exists for render :partial

actually, locals is accepted when rendering a template as well as a partial - but not for rendering an action, apparently. as reference, from the api docs:

Renders the template with a local variable

render :template => "weblog/show", :locals => {:customer => Customer.new}

i did get render_to_string working with local variables when i render a template, which is fine for my needs. not sure what the logic is behind allowing locals for rendering a template but not an action...