Caching problem in PackageBranch:cached_available_updates

[mkuron]

The first request for the Available Package Updates widget after restarting the server works fine:

Started GET "/dashboard/widget/available_package_updates?_=1345806177829" for 127.0.0.1 at 2012-08-24 13:02:57 +0200
Processing by DashboardController#widget as JS
  Parameters: {"_"=>"1345806177829", "name"=>"available_package_updates"}
  Rendered widgets/_available_package_updates.html.erb (1895.5ms)
  Rendered dashboard/widget.js.erb (1898.0ms)
Completed 200 OK in 1934ms (Views: 1875.9ms | ActiveRecord: 31.1ms)

Subsequent requests however return a HTTP 500 error:

Started GET "/dashboard/widget/available_package_updates?_=1345806374208" for 127.0.0.1 at 2012-08-24 13:06:14 +0200
Processing by DashboardController#widget as JS
  Parameters: {"_"=>"1345806374208", "name"=>"available_package_updates"}
  Rendered widgets/_available_package_updates.html.erb (12.0ms)
  Rendered dashboard/widget.js.erb (12.6ms)
Completed 500 Internal Server Error in 37ms

ActionView::Template::Error (undefined method `icon' for :@relation:Symbol):
    11:                     <% packages.each do |package| %>
    12:                         <p>
    13:                             <%= image_tag(package.icon.url(:tiny), :class => "thumbnail", "data-package-branch-id" => package.package_branch_id) if package.icon %>
    14:                             <%= link_to "#{package.name}", package_path(package.to_params) %> 
    15:                             version 
    16:                             <%= link_to package.package_branch.version_tracker.version, macupdate_url(package) %> is now available!
  app/views/widgets/_available_package_updates.html.erb:13:in `block (2 levels) in _app_views_widgets__available_package_updates_html_erb___1114117588481770231_2507639700'
  app/views/widgets/_available_package_updates.html.erb:11:in `each'
  app/views/widgets/_available_package_updates.html.erb:11:in `block in _app_views_widgets__available_package_updates_html_erb___1114117588481770231_2507639700'
  app/views/widgets/_available_package_updates.html.erb:1:in `each'
  app/views/widgets/_available_package_updates.html.erb:1:in `_app_views_widgets__available_package_updates_html_erb___1114117588481770231_2507639700'
  app/views/dashboard/widget.js.erb:1:in `_app_views_dashboard_widget_js_erb___17472507496854109_2172550500'
  app/controllers/dashboard_controller.rb:10:in `widget'

I added a line containing logger.warn packages to app/views/widgets/_available_package_updates.html.erb to see where it's going wrong. On the first run, packages is [#<Package id: 10 ...>, #<Package id: 13 ...>, #<Package id: 21 ...>], on subsequent runs it's [#<Package id: 10 ...>, :@relation, nil].
Apparently there is some caching that isn't doing what it should. Indeed, if I go into app/models/package_branch.rb and comment out lines 173 and 175 to disable caching, everything works as it should (albeit slower).