Use `Mongoid::Matchers#matches?` instead of a database query in...

Use `Mongoid::Matchers#matches?` instead of  a database query in `MongoidAdapter#matches_conditions_hash?`

lib/cancan/model_adapters/mongoid_adapter.rb

@@ -10,7 +10,9 @@ module CanCan
       end
 
       def self.matches_conditions_hash?(subject, conditions)
-        subject.class.where(conditions).include?(subject) # TODO don't use a database query here for performance and other instances
+        # To avoid hitting the db, retrieve the raw Mongo selector from
+        # the Mongoid Criteria and use Mongoid::Matchers#matches?
+        subject.matches?( subject.class.where(conditions).selector )
       end
 
       def database_records

spec/cancan/model_adapters/mongoid_adapter_spec.rb

@@ -84,7 +84,7 @@ if ENV["MODEL_ADAPTER"] == "mongoid"
           it "Calls where on the model class when there are criteria" do
             obj = MongoidProject.create(:title => 'Bird')
             @conditions = {:title.nin => ["Fork", "Spoon"]}
-            mock(MongoidProject).where(@conditions) {[obj]}
+
             @ability.can :read, MongoidProject, @conditions
             @ability.should be_able_to(:read, obj)
           end