class ActionController::Renderer

def render(*args)

Renders a template to a string, just like ActionController::Rendering#render_to_string. 
def render(*args)
  request = ActionDispatch::Request.new(env_for_request)
  request.routes = controller._routes
  instance = controller.new
  instance.set_request! request
  instance.set_response! controller.make_response!(request)
  instance.render_to_string(*args)
end