Train wreck getting render_to_string to work from foreign controller

I am calling render_to_string on an action of a different controller. I am not sure if this is just in Rails 3 (I am using) or if it would have presented in previous versions. I especially don’t understand why this method should ignore :controller => “xx” and assume the current controller.
Here are a few of the trials and results:
html_to_save = render_to_string(:action => “/comparisons#display”, :layout => “application”)
returns (in addition to a deprication error):
ActionView::MissingTemplate: Missing template /comparisons#display with {:handlers=>[:erb, :rjs, :builder, :rhtml, :rxml], :formats=>[:html], :locale=>[:en, :en]} in view paths “/Users/DK/Documents/ror/projects/creditcompare3/app/views”
**html_to_save = render_to_string(:action => “/comparisons/display”, :layout => “application”)**returns:
ActionView::Template::Error: Missing partial web_requests/display with {:handlers=>[:erb, :rjs, :builder, :rhtml, :rxml], :formats=>[:html], :locale=>[:en, :en]} in view paths “/Users/DK/Documents/ror/projects/creditcompare3/app/views”
html_to_save = render_to_string(:controller => “comparisons”, :action => “display”, :layout => “application”)
returns
:
ActionView::MissingTemplate: Missing template web_requests/display with {:handlers=>[:erb, :rjs, :builder, :rhtml, :rxml], :formats=>[:html], :locale=>[:en, :en]} in view paths “/Users/DK/Documents/ror/projects/creditcompare3/app/views”
html_to_save = render_to_string(:action => “display”, :layout => “application”)
returns
(obviously):
ActionView::MissingTemplate: Missing template web_requests/display with {:handlers=>[:erb, :rjs, :builder, :rhtml, :rxml], :formats=>[:html], :locale=>[:en, :en]} in view paths “/Users/DK/Documents/ror/projects/creditcompare3/app/views”

In the Rails source I find:

# Raw rendering of a template to a string. Just convert the results of
# render_to_body into a String.
# :api: plugin
def render_to_string(*args, &block)
  options = _normalize_args(*args, &block)
  _normalize_options(options)
  render_to_body(options)
end

# Raw rendering of a template to a Rack-compatible body.
# :api: plugin
def render_to_body(options = {})
  _process_options(options)
  _render_template(options)
end

# Find and renders a template based on the options given.
# :api: private
def _render_template(options) #:nodoc:
  view_context.render(options)
end

So I new to reading the source but is the problem that it is using the view_context which I would assume would be that which the current controller owns? That would seems to confirm the errors and ignorance of the controller attribute.

Thanks,

David