Rails Authorization with CanCan Problem

Question

On my rails app I have implemented AuthLogic and CanCan. However when trying to figure out if the user can manage an article (checks if he is owner through the article.user_id) with CanCan I am running into issues. This should be straight forward I don't know what I'm doing wrong.

User has_many Articles

class Ability
    include CanCan::Ability

    def initialize(user)
        user ||= User.new

        can :read, :all
        can :manage, Article do |article|
           article.user_id == user.id
        end
    end         
end

I get the following error when going to /articles/index

undefined method `user' for #<Array:0x1035c5158>
Extracted source (around line #15):

12:         <td><%= article.created_at %></td>
13:         <td><%= article.user.login %></td>  
14:         <td><%= link_to 'View', article %></td>
15:     <% if can? :manage, article %>  
16:         <td><%= link_to 'Edit', edit_article_path(article) %></td>
17:     <% end %>
18:     <% if can? :manage, article %>  

app/models/ability.rb:9:in `try'
app/models/ability.rb:9:in `initialize'
cancan (1.3.4) lib/cancan/can_definition.rb:101:in `call'
cancan (1.3.4) lib/cancan/can_definition.rb:101:in `call_block'
cancan (1.3.4) lib/cancan/can_definition.rb:29:in `matches_conditions?'
cancan (1.3.4) lib/cancan/ability.rb:54:in `can?'
authlogic (2.1.6) lib/authlogic/session/password.rb:127:in `detect'
cancan (1.3.4) lib/cancan/ability.rb:53:in `each'
cancan (1.3.4) lib/cancan/ability.rb:53:in `detect'
cancan (1.3.4) lib/cancan/ability.rb:53:in `can?'
cancan (1.3.4) lib/cancan/controller_additions.rb:228:in `can?'
actionpack (3.0.0) lib/abstract_controller/helpers.rb:55:in `send'
actionpack (3.0.0) lib/abstract_controller/helpers.rb:55:in `can?'
app/views/articles/_articles.html.erb:15:in `_app_views_articles__articles_html_erb__1624837607_2174751840_1924990'
app/views/articles/_articles.html.erb:8:in `each'
app/views/articles/_articles.html.erb:8:in `_app_views_articles__articles_html_erb__1624837607_2174751840_1924990'
actionpack (3.0.0) lib/action_view/template.rb:135:in `send'
actionpack (3.0.0) lib/action_view/template.rb:135:in `render'
activesupport (3.0.0) lib/active_support/notifications.rb:54:in `instrument'
actionpack (3.0.0) lib/action_view/template.rb:127:in `render'
actionpack (3.0.0) lib/action_view/render/partials.rb:294:in `render_partial'
actionpack (3.0.0) lib/action_view/render/partials.rb:223:in `render'
activesupport (3.0.0) lib/active_support/notifications.rb:52:in `instrument'
activesupport (3.0.0) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
activesupport (3.0.0) lib/active_support/notifications.rb:52:in `instrument'
actionpack (3.0.0) lib/action_view/render/partials.rb:221:in `render'
actionpack (3.0.0) lib/action_view/render/partials.rb:334:in `_render_partial'
actionpack (3.0.0) lib/action_view/render/rendering.rb:31:in `render'
app/views/articles/index.html.erb:5:in `_app_views_articles_index_html_erb___1319251192_2174766940_0'
actionpack (3.0.0) lib/action_view/template.rb:135:in `send'
actionpack (3.0.0) lib/action_view/template.rb:135:in `render'
activesupport (3.0.0) lib/active_support/notifications.rb:54:in `instrument'
actionpack (3.0.0) lib/action_view/template.rb:127:in `render'
actionpack (3.0.0) lib/action_view/render/rendering.rb:59:in `_render_template'
activesupport (3.0.0) lib/active_support/notifications.rb:52:in `instrument'
activesupport (3.0.0) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
activesupport (3.0.0) lib/active_support/notifications.rb:52:in `instrument'
actionpack (3.0.0) lib/action_view/render/rendering.rb:56:in `_render_template'
actionpack (3.0.0) lib/action_view/render/rendering.rb:26:in `render'
actionpack (3.0.0) lib/abstract_controller/rendering.rb:114:in `_render_template'
actionpack (3.0.0) lib/abstract_controller/rendering.rb:108:in `render_to_body'
actionpack (3.0.0) lib/action_controller/metal/renderers.rb:47:in `render_to_body'
actionpack (3.0.0) lib/action_controller/metal/compatibility.rb:55:in `render_to_body'
actionpack (3.0.0) lib/abstract_controller/rendering.rb:101:in `render_to_string'
actionpack (3.0.0) lib/abstract_controller/rendering.rb:92:in `render'
actionpack (3.0.0) lib/action_controller/metal/rendering.rb:17:in `render'
actionpack (3.0.0) lib/action_controller/metal/instrumentation.rb:40:in `render'
activesupport (3.0.0) lib/active_support/core_ext/benchmark.rb:5:in `ms'
/opt/local/lib/ruby/1.8/benchmark.rb:308:in `realtime'
activesupport (3.0.0) lib/active_support/core_ext/benchmark.rb:5:in `ms'
actionpack (3.0.0) lib/action_controller/metal/instrumentation.rb:40:in `render'
actionpack (3.0.0) lib/action_controller/metal/instrumentation.rb:78:in `cleanup_view_runtime'
activerecord (3.0.0) lib/active_record/railties/controller_runtime.rb:15:in `cleanup_view_runtime'
actionpack (3.0.0) lib/action_controller/metal/instrumentation.rb:39:in `render'
actionpack (3.0.0) lib/action_controller/metal/implicit_render.rb:10:in `default_render'
actionpack (3.0.0) lib/action_controller/metal/implicit_render.rb:5:in `send_action'
actionpack (3.0.0) lib/abstract_controller/base.rb:150:in `process_action'
actionpack (3.0.0) lib/action_controller/metal/rendering.rb:11:in `process_action'
actionpack (3.0.0) lib/abstract_controller/callbacks.rb:18:in `process_action'
activesupport (3.0.0) lib/active_support/callbacks.rb:440:in `_run__704842227__process_action__199225275__callbacks'
activesupport (3.0.0) lib/active_support/callbacks.rb:409:in `send'
activesupport (3.0.0) lib/active_support/callbacks.rb:409:in `_run_process_action_callbacks'
activesupport (3.0.0) lib/active_support/callbacks.rb:93:in `send'
activesupport (3.0.0) lib/active_support/callbacks.rb:93:in `run_callbacks'
actionpack (3.0.0) lib/abstract_controller/callbacks.rb:17:in `process_action'
actionpack (3.0.0) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
activesupport (3.0.0) lib/active_support/notifications.rb:52:in `instrument'
activesupport (3.0.0) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
activesupport (3.0.0) lib/active_support/notifications.rb:52:in `instrument'
actionpack (3.0.0) lib/action_controller/metal/instrumentation.rb:29:in `process_action'
actionpack (3.0.0) lib/action_controller/metal/rescue.rb:17:in `process_action'
actionpack (3.0.0) lib/abstract_controller/base.rb:119:in `process'
actionpack (3.0.0) lib/abstract_controller/rendering.rb:40:in `process'
actionpack (3.0.0) lib/action_controller/metal.rb:133:in `dispatch'
actionpack (3.0.0) lib/action_controller/metal/rack_delegation.rb:14:in `dispatch'
actionpack (3.0.0) lib/action_controller/metal.rb:173:in `action'
actionpack (3.0.0) lib/action_dispatch/routing/route_set.rb:62:in `call'
actionpack (3.0.0) lib/action_dispatch/routing/route_set.rb:62:in `dispatch'
actionpack (3.0.0) lib/action_dispatch/routing/route_set.rb:27:in `call'
rack-mount (0.6.13) lib/rack/mount/route_set.rb:148:in `call'
rack-mount (0.6.13) lib/rack/mount/code_generation.rb:93:in `recognize'
rack-mount (0.6.13) lib/rack/mount/code_generation.rb:68:in `optimized_each'
rack-mount (0.6.13) lib/rack/mount/code_generation.rb:92:in `recognize'
rack-mount (0.6.13) lib/rack/mount/route_set.rb:139:in `call'
actionpack (3.0.0) lib/action_dispatch/routing/route_set.rb:492:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/best_standards_support.rb:17:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/head.rb:14:in `call'
rack (1.2.1) lib/rack/methodoverride.rb:24:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/params_parser.rb:21:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/flash.rb:182:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/session/abstract_store.rb:149:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/cookies.rb:287:in `call'
activerecord (3.0.0) lib/active_record/query_cache.rb:32:in `call'
activerecord (3.0.0) lib/active_record/connection_adapters/abstract/query_cache.rb:28:in `cache'
activerecord (3.0.0) lib/active_record/query_cache.rb:12:in `cache'
activerecord (3.0.0) lib/active_record/query_cache.rb:31:in `call'
activerecord (3.0.0) lib/active_record/connection_adapters/abstract/connection_pool.rb:355:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/callbacks.rb:46:in `call'
activesupport (3.0.0) lib/active_support/callbacks.rb:415:in `_run_call_callbacks'
actionpack (3.0.0) lib/action_dispatch/middleware/callbacks.rb:44:in `call'
rack (1.2.1) lib/rack/sendfile.rb:107:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/remote_ip.rb:48:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/show_exceptions.rb:46:in `call'
railties (3.0.0) lib/rails/rack/logger.rb:13:in `call'
rack (1.2.1) lib/rack/runtime.rb:17:in `call'
activesupport (3.0.0) lib/active_support/cache/strategy/local_cache.rb:72:in `call'
rack (1.2.1) lib/rack/lock.rb:11:in `call'
rack (1.2.1) lib/rack/lock.rb:11:in `synchronize'
rack (1.2.1) lib/rack/lock.rb:11:in `call'
actionpack (3.0.0) lib/action_dispatch/middleware/static.rb:30:in `call'
railties (3.0.0) lib/rails/application.rb:168:in `call'
railties (3.0.0) lib/rails/application.rb:77:in `send'
railties (3.0.0) lib/rails/application.rb:77:in `method_missing'
railties (3.0.0) lib/rails/rack/log_tailer.rb:14:in `call'
rack (1.2.1) lib/rack/content_length.rb:13:in `call'
rack (1.2.1) lib/rack/handler/webrick.rb:52:in `service'
/opt/local/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
/opt/local/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
/opt/local/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
/opt/local/lib/ruby/1.8/webrick/server.rb:162:in `start'
/opt/local/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
/opt/local/lib/ruby/1.8/webrick/server.rb:95:in `start'
/opt/local/lib/ruby/1.8/webrick/server.rb:92:in `each'
/opt/local/lib/ruby/1.8/webrick/server.rb:92:in `start'
/opt/local/lib/ruby/1.8/webrick/server.rb:23:in `start'
/opt/local/lib/ruby/1.8/webrick/server.rb:82:in `start'
rack (1.2.1) lib/rack/handler/webrick.rb:13:in `run'
rack (1.2.1) lib/rack/server.rb:213:in `start'
railties (3.0.0) lib/rails/commands/server.rb:65:in `start'
railties (3.0.0) lib/rails/commands.rb:30
railties (3.0.0) lib/rails/commands.rb:27:in `tap'
railties (3.0.0) lib/rails/commands.rb:27
script/rails:6:in `require'
script/rails:6

Here is my view:

<% if can? :manage, article %>  
    <td><%= link_to 'Edit', edit_article_path(article) %></td>
<% end %>
<% if can? :manage, article %>  
    <td><%= link_to 'Delete', article, :method => :delete %></td>
<% end %>

Here is my controller:

  def index
    @articles = Article.find(:all)      
  end

Answer 1

Not sure exactly what your problem cause is without a trace, but I find I often have to check for nil when using a value, i.e. in your example:

can :manage, Article do |article|
  article && article.user_id == user.id
end

it seems that in certain situations the block argument is nil

According to http://github.com/ryanb/cancan/wiki/defining-abilities-with-hashes there may be a cleaner way:

can :manage, Article, :user_id => user.id

I would try that method first, see how you go.