Fixed bug where conditions on an optionally associated object would throw exceptions if the associated object was not present at the rule match time.

lib/cancan/rule.rb

@@ -109,7 +109,7 @@ module CanCan
                 if attribute.kind_of? Array
                   attribute.any? { |element| matches_conditions_hash? element, value }
                 else
-                  matches_conditions_hash? attribute, value
+                  !attribute.nil? && matches_conditions_hash?(attribute, value)
                 end
               elsif value.kind_of?(Array) || value.kind_of?(Range)
                 value.include? attribute

spec/cancan/ability_spec.rb

@@ -250,6 +250,13 @@ describe CanCan::Ability do
     @ability.can?(:read, 4..6).should be_false
   end
   
+  it "should not match subjects return nil for methods that must match nested a nested conditions hash" do
+    mock(object_with_foo = Object.new).foo { :bar }
+    @ability.can :read, Array, :first => { :foo => :bar }
+    @ability.can?(:read, [object_with_foo]).should be_true
+    @ability.can?(:read, []).should be_false
+  end
+
   it "should not stop at cannot definition when comparing class" do
     @ability.can :read, Range
     @ability.cannot :read, Range, :begin => 1