Rails Routing to grandparent nested class - ruby-on-rails

Question

I'm having a problem with a nested routing when I try to edit the subresource (lessons) from it's show page by clicking the edit link as described below. There are two classes involved: Units and Lessons. I can't understand why Rails is trying to route to course controller (a parent of Units).
Any help would be greatly appreciated.
I am getting this error and url
http://localhost:3000/units/3/lessons/13/edit
Routing Error
No route matches {:action=>"show", :controller=>"courses", :id=>nil}
config/routes.rb
resources :courses do
resources :units
end
resources :units do
resources :lessons
end
** from rake routes**
unit_lessons GET /units/:unit_id/lessons(.:format) lessons#index
POST /units/:unit_id/lessons(.:format) lessons#create
new_unit_lesson GET /units/:unit_id/lessons/new(.:format) lessons#new
edit_unit_lesson GET /units/:unit_id/lessons/:id/edit(.:format) lessons#edit
unit_lesson GET /units/:unit_id/lessons/:id(.:format) lessons#show
code for clicked link
<%= link_to "Edit Lesson", edit_unit_lesson_path(#unit, #lesson ) %>
LessonsController
class LessonsController < ApplicationController
before_filter :find_unit
before_filter :find_lesson, :only => [:show,:edit,:update,:destroy]
.
.
.
private
def find_unit
#unit = Unit.find(params[:unit_id])
end
def find_lesson
#lesson = #unit.lessons.find(params[:id])
end
Server Log File
Started GET "/units/3/lessons/12/edit" for 127.0.0.1 at 2012-08-02 14:50:55 +0200
Processing by LessonsController#edit as HTML
Parameters: {"unit_id"=>"3", "id"=>"12"}
Unit Load (0.1ms) SELECT "units".* FROM "units" WHERE "units"."id" = ? LIMIT 1 [["id", "3"]]
Lesson Load (0.1ms) SELECT "lessons".* FROM "lessons" WHERE "lessons"."unit_id" = 3 AND "lessons"."id" = ? LIMIT 1 [["id", "12"]]
Rendered lessons/_form.html.erb (3.4ms)
Rendered lessons/edit.html.erb within layouts/application (4.4ms)
Completed 500 Internal Server Error in 7ms
ActionController::RoutingError (No route matches {:action=>"show", :controller=>"courses", :id=>nil}):
app/views/lessons/edit.html.erb:8:in `_app_views_lessons_edit_html_erb___3830769233446763788_70188197130200'

Answer 1

Found it. The link was part of a table body, built with an .each iterator. I needed to change the name from "#lesson" to "lesson" as that is what is referenced when the table is built.
For anyone else stuck on a similar problem, I've put the functioning code below.
<tbody>
<% #unit.lessons.each do |lesson| %>
<tr>
<td><%= link_to lesson.lesson_name, [#unit, lesson] %></td>
.
.
.
<td><%= link_to 'Edit', edit_unit_lesson_path(#unit, lesson ) %></td>
</tr>
<% end %>
</tbody>