Load XML into variable with Rails

This is how I generate XML for purchase model:

# GET /purchases/1
def show
  @purchase = Purchase.find(params[:id])
  @purchases = Purchase.all

  respond_to do |format|
    format.html # show.html.erb
    format.xml  { render :action => "something.xml.builder", :layout => false }
  end
end

Now I'd like to get this rendered XML as string into variable so I could post it to WebService.

How can I get XML through sales_invoice.xml.builder with开发者_StackOverflow社区out rendering it? I don't want use dirty hacks and loading XML from http://appurl/purchases/1.xml

Thanks!

What I was looking for was render_to_string method.