Find_commentable return undefined method `comments' for nil: NilClass

I keep getting undefined method 'comments' for nil:NilClassin my create method when I try to create a new comment (polymorphic relationship). I looked through a few other questions about this and cannot find what is wrong with my form / controller.

Here is my general partial comment:

<%= form_for [@commentable, Comment.new] do |f| %>
  <%= f.text_area :content %>
  <%= f.submit "Post" %>
<% end %>

Please note that this appears on my traveldeal / show page. The shape looks great. If I change form_for to pass parameters [@commentable, @comment], I get the undefined methodmodel_name ' error for NilClass: Class`

routes.rb

resources :users 
resources :traveldeals

resources :traveldeals do
  resources :comments
end

resources :users do
  resources :comments
end

Railscasts says above as resources :traveldeals, :has_many => :comments, but I believe this is a dated syntax.

comments_controller.rb

class CommentsController < ApplicationController
  before_filter :authenticate, :only => [:create, :destroy]

  def new
    @comment = Comment.new
  end

  def create
    @commentable = find_commentable
    @comment = @commentable.comments.build(params[:comment])
    @comment.user_id = current_user.id

   if @comment.save
      flash[:success] = "Successfully saved comment."  
      redirect_to root_path
    else
      redirect_to current_user
    end
  end

private

  def find_commentable
    params.each do |name, value|
      if name =~ /(.+)_id$/
        return $1.classify.constantize.find(value)
      end
    end
    nil
   end

end

Edit: Added solution, so the above code works for me.