load member through method instead of instance variable to improve decent_exposure support

lib/cancan/controller_resource.rb

@@ -148,7 +148,13 @@ module CanCan
     end
 
     def resource_instance
-      @controller.instance_variable_get("@#{instance_name}") if load_instance?
+      if load_instance?
+        if @controller.instance_variable_defined? "@#{instance_name}"
+          @controller.instance_variable_get("@#{instance_name}")
+        elsif @controller.respond_to?(instance_name, true)
+          @controller.send(instance_name)
+        end
+      end
     end
 
     def collection_instance=(instance)

spec/cancan/controller_resource_spec.rb

@@ -336,6 +336,14 @@ describe CanCan::ControllerResource do
     CanCan::ControllerResource.new(@controller, :authorize => true).process
   end
 
+  it "should fetch member through method when instance variable is not provided" do
+    stub(@controller).project { :some_project }
+    @params.merge!(:action => "show", :id => 123)
+    stub(@controller).authorize!(:show, :some_project) { raise CanCan::Unauthorized }
+    resource = CanCan::ControllerResource.new(@controller, :authorize => true)
+    lambda { resource.process }.should raise_error(CanCan::Unauthorized)
+  end
+
   # it "should raise ImplementationRemoved when adding :name option" do
   #   lambda {
   #     CanCan::ControllerResource.new(@controller, :name => :foo)