Undefined method `each' for nil:NilClass in a partial - ruby-on-rails

Question

I am trying to display a collection as a partial ( on a side panel). I get an error above, have been fighting for several hours and do not understand what is the problem.
Containing view (a partial template included into application.html.haml file):
%span.span12
%ul.nav.nav-pills.nav-stacked{style: "color: white;"}
= render :partial => 'categories/categories_list'
Partial /categories/_category_list.html.haml:
%ul
- #categories.each do |category|
%li{style: "color: white;"}=category.name
Controller (categories_controller):
class Macro::CategoriesController < ApplicationController
def categories_list
#categories= Category.all
end
end
I have separately tried to add "collection: #categories" as an option for render :partial - to non avail. When I replaced the instance variable with just an array in order to test the partial rendering, it worked ( the list items got rendered). Seem like the variable #categories does not get instantiated, however I have double-checked in console that a given activerecord query returns a set of objects. I am breaking my brain to see where I made a mistake.
Log:
Started GET "/ios/macro/namespace2/another" for 127.0.0.1 at 2014-02-11 11:41:23 +0100
Processing by Macro::Namespace2::AnotherController#index as HTML
Parameters: {"param1"=>"xx"}
....... (SQL queries for that view)
Rendered macro/namespace2/another_controller/_another_view.html.haml (7.0ms)
Rendered macro/namespace2/another_controller/index.html.haml within layouts/application (29.6ms)
Rendered layouts/_logout_link.html.haml (0.5ms)
Rendered layouts/_top_navigation.html.haml (3.3ms)
Rendered layouts/_search.html.haml (0.8ms)
Rendered macro/categories/_categories_list.html.haml (0.8ms)
Rendered macro/_navigation.html.haml (3.6ms)
Completed 500 Internal Server Error in 49ms
ActionView::Template::Error (undefined method `each' for nil:NilClass):
1: %ul
2: - #categories.each do |category|
3: %li{style: "color: white;"}=category
app/views/macro/categories/_categories_list.html.haml:2:in `_app_views_macro_categories__categories_list_html_haml__246519878332953677_70248836672780'
app/views/macro/_navigation.html.haml:11:in `_app_views_macro__navigation_html_haml__3071184578230019020_70248793690940'
app/views/layouts/application.html.haml:24:in `_app_views_layouts_application_html_haml__3208937943371424843_70248793607300'
Rendered /Users/me/.rvm/gems/ruby-2.0.0-p353/gems/actionpack-4.0.0/lib/action_dispatch/middleware/templates/rescues/_trace.erb (1.1ms)
Rendered /Users/me/.rvm/gems/ruby-2.0.0-p353/gems/actionpack-4.0.0/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb (0.8ms)
Rendered /Users/me/.rvm/gems/ruby-2.0.0-p353/gems/actionpack-4.0.0/lib/action_dispatch/middleware/templates/rescues/template_error.erb within rescues/layout (10.6ms)
Routes:
namespace "macro" do
namespace "namespace2" do
resources :another, only: [:index]
end
resources :categories
end
end

Answer 1

Probably the #categories_list action in Macro::CategoriesController is not being called. Check the log and see which controller and action is handling the request.
You should see something like:
'Processing by Macro::CategoriesController#categories_list as HTML'
'Parameters: { }'