Fix accessible_by for Mongoid documents when no ability is defined.

The previous spec that checked for this was not right, since there were no documents in the collection, so every query would return an empty result.

lib/cancan/mongoid_additions.rb

@@ -86,7 +86,7 @@ module CanCan
         if query.conditions.blank?
           # this query is sure to return no results
           # we need this so there is a Mongoid::Criteria object to return, since an empty array would cause problems
-          where({:_id => {'$exists' => true, '$type' => 2}})  
+          where({:_id => {'$exists' => false, '$type' => 7}})  # type 7 is an ObjectID (default for _id)
         else   
           where(query.conditions)
         end

spec/cancan/mongoid_additions_spec.rb

@@ -51,6 +51,10 @@ describe CanCan::MongoidAdditions do
   end
 
   it "should return [] when no ability is defined so no records are found" do
+    @model_class.create :title  => 'Sir'
+    @model_class.create :title  => 'Lord'
+    @model_class.create :title  => 'Dude'
+      
     @model_class.accessible_by(@ability, :read).should == []
   end