Strange unexplainable error

In one of my Controllers I stumbled upon a really strange error. On render :action => :create I get this error message:

NoMethodError in Admin/machines/baseController#save undefined method `to_str' for [:action, :create]:Array

Application Trace: /usr/local/lib/ruby/gems/1.8/gems/map_by_method-0.7.0/lib/ map_by_method.rb:12:in `method_missing' vendor/rails/actionpack/lib/action_controller/layout.rb:241:in `to_s' vendor/rails/actionpack/lib/action_controller/layout.rb:241:in `render_without_benchmark' vendor/rails/actionpack/lib/action_controller/benchmarking.rb:50:in `render' /usr/local/lib/ruby/1.8/benchmark.rb:293:in `measure' vendor/rails/actionpack/lib/action_controller/benchmarking.rb:50:in `render' app/controllers/admin/machines/base_controller.rb:22:in `save'

I've done rendering actions probably a hundred times before and I don't see anything wrong with my code. But just for good measure here's the code I'm using. When saving fails because of some validation errors it is supposed to render the create view again but dies with the above error message.

class Admin::Machines::BaseController < ApplicationController   def create     @machine = Machine::Base.new     @picture = Picture.new   end

  def save     @machine = Machine::Base.new(params[:machine])     @picture = Picture.new(params[:picture])     @machine.picture = @picture     if @machine.save && @picture.save       flash[:notice] = "yay!"       redirect_to :action => :index     else       flash[:error] = "nay"       render :action => :create     end   end end

Cheers Martin

Nevermind, it turned out to be a problem with map_by_method as described at http://drnicwilliams.com/2007/08/12/map_by_method-now-works-with-activerecord-associations/ (comment 8 and following)