inconsistent behavior between permitted_to? and Model.with_permissions_to

[cschoene]

I just ran into an issue with Model.with_permissions_to:

authorization_rules.rb:

has_permission_on :chats do
  to :subscribe
  if_attribute :activity => {:parent => {:participants => contains {user}}}
end  

The rule works as expected when checking permissions on an object with permitted_to?(:subscribe, chatObj), however, using Chat.with_permissions_to(:subscribe) produces an error:

Loading development environment (Rails 3.2.1)
1.9.3-p0 :001 > Authorization.current_user = User.first
(3.5ms) SELECT * FROM geometry_columns WHERE f_table_name='users'
User Load (1.0ms) SELECT "users".* FROM "users" LIMIT 1
=> #<User id: 14, ....>
1.9.3-p0 :002 > ch = Chat.with_permissions_to(:subscribe)
(2.4ms) SELECT * FROM geometry_columns WHERE f_table_name='roles'
Role Load (0.9ms) SELECT "roles".* FROM "roles" WHERE "roles"."id" = 7 LIMIT 1
steps: {:participants=>[:contains, #<User id: 14,...>]}
value: nil
NoMethodError: undefined method length' for nil:NilClass from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/obligation_scope.rb:122:infollow_comparison'
from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/obligation_scope.rb:87:in block in follow_path' from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/obligation_scope.rb:81:ineach'
from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/obligation_scope.rb:81:in follow_path' from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/obligation_scope.rb:85:inblock in follow_path'
from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/obligation_scope.rb:81:in each' from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/obligation_scope.rb:81:infollow_path'
from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/obligation_scope.rb:69:in parse!' from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/in_model.rb:76:inblock in obligation_scope_for'
from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/in_model.rb:75:in each' from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/in_model.rb:75:inobligation_scope_for'
from .rvm/gems/ruby-1.9.3-p0/gems/declarative_authorization-0.5.6/lib/declarative_authorization/in_model.rb:124:in with_permissions_to' from (irb):2 from .rvm/gems/ruby-1.9.3-p0/gems/railties-3.2.1/lib/rails/commands/console.rb:47:instart'
from .rvm/gems/ruby-1.9.3-p0/gems/railties-3.2.1/lib/rails/commands/console.rb:8:in start' from .rvm/gems/ruby-1.9.3-p0/gems/railties-3.2.1/lib/rails/commands.rb:41:in<top (required)>'
from script/rails:6:in require' from script/rails:6:in

'1.9.3-p0 :003 >

I output "steps:" and "value:" in obligation_scope.rb like this:

def follow_comparison( steps, past_steps, attribute )
  operator = steps[0]
  puts "steps: #{steps.inspect}"
  value = steps[1..-1]
  puts "value: #{value.inspect}"
  value = value[0] if value.length == 1

  add_obligation_condition_for( past_steps, [attribute, operator, value] )
end

Possibly the same problem as described in:
http://stackoverflow.com/questions/15211854/declarative-authorization-if-permitted-to-with-polymorphic-association-fail