Adds ability to use Scope query with Mongoid

Same limitations apply as with active record
* can not be OR'd with other rules for same ability/controller

lib/cancan/model_adapters/mongoid_adapter.rb

@@ -18,6 +18,8 @@ module CanCan
       def database_records
         if @rules.size == 0  
           @model_class.where(:_id => {'$exists' => false, '$type' => 7}) # return no records in Mongoid
+        elsif @rules.size == 1 && @rules[0].conditions.is_a?(Mongoid::Criteria)
+          @rules[0].conditions
         else
           @rules.inject(@model_class.all) do |records, rule|
             if rule.conditions.empty?
@@ -37,4 +39,4 @@ end
 # simplest way to add `accessible_by` to all Mongoid Documents
 module Mongoid::Document::ClassMethods
   include CanCan::ModelAdditions::ClassMethods
-end
+end