> Spree::UserAuthentication.all.select{|v| v.user.nil? }
=> [#<Spree::UserAuthentication id: 5, user_id: 123, provider: "facebook", uid: "xxxxxxxxxx", created_at: "2012-07-19 10:07:34", updated_at: "2012-07-19 10:07:34">, #<Spree::UserAuthentication id: 6, user_id: 512, provider: "google_oauth2", uid: "xxxxxxxxxx", created_at: "2012-07-19 10:15:55", updated_at: "2012-07-19 10:15:55">]
[GEM_ROOT]/gems/devise-2.0.4/lib/devise/rails/warden_compat.rb:19:in `serialize'
[GEM_ROOT]/gems/warden-1.1.1/lib/warden/session_serializer.rb:25:in `store'
[GEM_ROOT]/gems/warden-1.1.1/lib/warden/proxy.rb:161:in `set_user'
[GEM_ROOT]/gems/devise-2.0.4/lib/devise/controllers/helpers.rb:124:in `sign_in'
[GEM_ROOT]/gems/devise-2.0.4/lib/devise/controllers/helpers.rb:236:in `sign_in_and_redirect'
(eval):13:in `facebook'
[GEM_ROOT]/gems/actionpack-3.2.6/lib/action_controller/metal/implicit_render.rb:4:in `send_action'
[GEM_ROOT]/gems/actionpack-3.2.6/lib/abstract_controller/base.rb:167:in `process_action'
[GEM_ROOT]/gems/actionpack-3.2.6/lib/action_controller/metal/rendering.rb:10:in `process_action'
[GEM_ROOT]/gems/actionpack-3.2.6/lib/abstract_controller/callbacks.rb:18:in `block in process_action'
[GEM_ROOT]/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:524:in `_run__271410053019927760__process_action__2888712652450066131__callbacks'
[GEM_ROOT]/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:405:in `__run_callback'
[GEM_ROOT]/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:385:in `_run_process_action_callbacks'
[GEM_ROOT]/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:81:in `run_callbacks'