If no decorator is found, fall back to decorators for parent classes

lib/active_decorator/decorator.rb

@@ -36,16 +36,21 @@ def to_a_with_decorator
     def decorator_for(model_class)
       return @@decorators[model_class] if @@decorators.has_key? model_class
 
-      decorator_name = "#{model_class.name}Decorator"
-      d = decorator_name.constantize
-      unless Class === d
-        d.send :include, ActiveDecorator::Helpers
-        @@decorators[model_class] = d
-      else
-        @@decorators[model_class] = nil
+      (model_class.ancestors - model_class.included_modules).find do |candidate|
+        decorator_name = "#{candidate.name}Decorator"
+
+        begin
+          d = decorator_name.constantize
+
+          unless Class === d
+            d.send :include, ActiveDecorator::Helpers
+            @@decorators[model_class] = d
+            return d
+          end
+        rescue NameError
+          next
+        end
       end
-    rescue NameError
-      @@decorators[model_class] = nil
     end
   end
 end

spec/decorator_spec.rb

@@ -0,0 +1,30 @@
+require 'spec_helper'
+
+describe ActiveDecorator::Decorator do
+  subject do
+    ActiveDecorator::Decorator.instance
+  end
+
+  describe '#decorator_for' do
+    context 'when a decorator exists' do
+      it 'returns the decorator' do
+        subject.send(:decorator_for, Parent).should == ParentDecorator
+      end
+    end
+
+    context 'when a parent decorator exists' do
+      it 'returns the decorator' do
+        subject.send(:decorator_for, Child).should == ParentDecorator
+      end
+    end
+  end
+end
+
+class Parent
+end
+
+class Child < Parent
+end
+
+module ParentDecorator
+end